[build-utils] Add flag for npm install --legacy-peer-deps (#8550)

Prior to `npm@8.6.0`, running `npm install --legacy-peer-deps` to generate a lock file and then later running `npm install` would work fine.

After `npm@8.6.0`, running `npm install` with that same lock file will fail with `code ERESOLVE` and `Conflicting peer dependency`.

Steps to reproduce:

```sh
corepack enable npm
echo '{"packageManager":"npm@8.5.5","dependencies":{"next":"12.3.0","react":"16.8.0"}}' > package.json
npm install --legacy-peer-deps
npm install # works
echo '{"packageManager":"npm@8.6.0","dependencies":{"next":"12.3.0","react":"16.8.0"}}' > package.json
npm install # fails with code ERESOLVE
```


This PR introduces a flag so we can safely upgrade npm to a newer version without breaking thousands of deployments.

The [npm docs](https://docs.npmjs.com/cli/v8/using-npm/config#legacy-peer-deps) say the flag is not recommended so we also make sure that once we upgrade to Node.js 18, we stop flagging `--legacy-peer-deps` because its safe to introduce breaking changes.

packages/build-utils/test/unit.run-npm-install.test.ts

@@ -0,0 +1,110 @@
+const spawnMock = jest.fn();
+jest.mock('cross-spawn', () => {
+  const spawn = (...args: any) => {
+    spawnMock(...args);
+    const child = {
+      on: (type: string, fn: (code: number) => void) => {
+        if (type === 'close') {
+          return fn(0);
+        }
+      },
+    };
+    return child;
+  };
+  return spawn;
+});
+
+afterEach(() => {
+  spawnMock.mockClear();
+});
+
+import path from 'path';
+import { runNpmInstall, cloneEnv } from '../src';
+
+function getTestSpawnOpts(env: Record<string, string>) {
+  return { env: cloneEnv(process.env, env) };
+}
+
+function getNodeVersion(major: number) {
+  return { major, range: `${major}.x`, runtime: `nodejs${major}.x` };
+}
+
+it('should not include peer dependencies when missing VERCEL_NPM_LEGACY_PEER_DEPS on node16', async () => {
+  const fixture = path.join(__dirname, 'fixtures', '20-npm-7');
+  const meta = {};
+  const spawnOpts = getTestSpawnOpts({});
+  const nodeVersion = { major: 16 } as any;
+  await runNpmInstall(fixture, [], spawnOpts, meta, nodeVersion);
+  expect(spawnMock.mock.calls.length).toBe(1);
+  const args = spawnMock.mock.calls[0];
+  expect(args[0]).toEqual('npm');
+  expect(args[1]).toEqual(['install', '--no-audit', '--unsafe-perm']);
+  expect(args[2]).toEqual({
+    cwd: fixture,
+    prettyCommand: 'npm install',
+    stdio: 'inherit',
+    env: expect.any(Object),
+  });
+});
+
+it('should include peer dependencies when VERCEL_NPM_LEGACY_PEER_DEPS=1 on node16', async () => {
+  const fixture = path.join(__dirname, 'fixtures', '20-npm-7');
+  const meta = {};
+  const spawnOpts = getTestSpawnOpts({ VERCEL_NPM_LEGACY_PEER_DEPS: '1' });
+  const nodeVersion = getNodeVersion(16);
+  await runNpmInstall(fixture, [], spawnOpts, meta, nodeVersion);
+  expect(spawnMock.mock.calls.length).toBe(1);
+  const args = spawnMock.mock.calls[0];
+  expect(args[0]).toEqual('npm');
+  expect(args[1]).toEqual([
+    'install',
+    '--no-audit',
+    '--unsafe-perm',
+    '--legacy-peer-deps',
+  ]);
+  expect(args[2]).toEqual({
+    cwd: fixture,
+    prettyCommand: 'npm install',
+    stdio: 'inherit',
+    env: expect.any(Object),
+  });
+});
+
+it('should not include peer dependencies when VERCEL_NPM_LEGACY_PEER_DEPS=1 on node14', async () => {
+  const fixture = path.join(__dirname, 'fixtures', '20-npm-7');
+  const meta = {};
+  const spawnOpts = getTestSpawnOpts({ VERCEL_NPM_LEGACY_PEER_DEPS: '1' });
+  const nodeVersion = getNodeVersion(14);
+  await runNpmInstall(fixture, [], spawnOpts, meta, nodeVersion);
+  expect(spawnMock.mock.calls.length).toBe(1);
+  const args = spawnMock.mock.calls[0];
+  expect(args[0]).toEqual('npm');
+  expect(args[1]).toEqual(['install', '--no-audit', '--unsafe-perm']);
+  expect(args[2]).toEqual({
+    cwd: fixture,
+    prettyCommand: 'npm install',
+    stdio: 'inherit',
+    env: expect.any(Object),
+  });
+});
+
+it('should not include peer dependencies when VERCEL_NPM_LEGACY_PEER_DEPS=1 on node16 with corepack enabled', async () => {
+  const fixture = path.join(__dirname, 'fixtures', '20-npm-7');
+  const meta = {};
+  const spawnOpts = getTestSpawnOpts({
+    VERCEL_NPM_LEGACY_PEER_DEPS: '1',
+    ENABLE_EXPERIMENTAL_COREPACK: '1',
+  });
+  const nodeVersion = getNodeVersion(16);
+  await runNpmInstall(fixture, [], spawnOpts, meta, nodeVersion);
+  expect(spawnMock.mock.calls.length).toBe(1);
+  const args = spawnMock.mock.calls[0];
+  expect(args[0]).toEqual('npm');
+  expect(args[1]).toEqual(['install', '--no-audit', '--unsafe-perm']);
+  expect(args[2]).toEqual({
+    cwd: fixture,
+    prettyCommand: 'npm install',
+    stdio: 'inherit',
+    env: expect.any(Object),
+  });
+});