[now-build-utils] Fix now dev to use system node (#3509)

This PR will use the system installed version of Node.js and avoid printing a warning or error if a discontinued version is selected.

This optimization was already in `@now/node` but for some reason it was never add to `@now/next`.

The reason why its relevant today is because the warnings turned into errors due to Node 8 deprecation and we don't have the "Project" in `now dev` so we don't know which version of node to select.

So instead of determining the version, `now dev` will always use `node` in the PATH and avoid printing warnings or errors. This also results in less FS reads since we no longer need to read package.json.

packages/now-build-utils/test/unit.test.js

@@ -104,6 +104,17 @@ it('should select correct node version in getNodeVersion()', async () => {
   ).toHaveProperty('major', 10);
 });
 
+it('should ignore node version in now dev getNodeVersion()', async () => {
+  expect(
+    await getNodeVersion(
+      '/tmp',
+      undefined,
+      { nodeVersion: '1' },
+      { isDev: true }
+    )
+  ).toHaveProperty('runtime', 'nodejs');
+});
+
 it('should get latest node version', async () => {
   expect(await getLatestNodeVersion()).toHaveProperty('major', 12);
 });