[tests] Migrate from yarn to pnpm (#9198)

<picture data-single-emoji=":pnpm:" title=":pnpm:"><img class="emoji" src="https://single-emoji.vercel.app/api/emoji/eyJhbGciOiJkaXIiLCJlbmMiOiJBMjU2R0NNIn0..4mJzrO94AnSn0Pue.4apgaKtTUdQ-wxNyahjdJj28u8bbXreLoTA8AGqYjLta3MrsFvbo9DsQFth4CoIkBgXFhQ5_BVcKNfYbwLg4bKzyIvItKe4OFS8AzG7Kkicz2kUUZk0.nXyK_PvHzZFGA-MQB6XHfA" alt=":pnpm:" width="20" height="auto" align="absmiddle"></picture> 

yarn has become increasingly more difficult to use as the v1 we rely on no longer receives updates. pnpm is faster and is actively maintained. 

This PR migrates us to pnpm.

packages/cli/scripts/build.ts

@@ -47,12 +47,12 @@ async function main() {
   const dependencies = Object.keys(pkg?.dependencies ?? {});
   // Do the initial `ncc` build
   console.log('Dependencies:', dependencies);
-  const externs = [];
+  const externs: Array<string> = [];
   for (const dep of dependencies) {
     externs.push('--external', dep);
   }
   const args = ['ncc', 'build', 'src/index.ts', ...externs];
-  await execa('yarn', args, { stdio: 'inherit', cwd: dirRoot });
+  await execa('pnpm', args, { stdio: 'inherit', cwd: dirRoot });
 
   // `ncc` has some issues with `@vercel/fun`'s runtime files:
   //   - Executable bits on the `bootstrap` files appear to be lost:
@@ -66,10 +66,7 @@ async function main() {
   // get compiled into the final ncc bundle file, however, we want them to be
   // present in the npm package because the contents of those files are involved
   // with `fun`'s cache invalidation mechanism and they need to be shasum'd.
-  const runtimes = join(
-    dirRoot,
-    '../../node_modules/@vercel/fun/dist/src/runtimes'
-  );
+  const runtimes = join(dirRoot, 'node_modules/@vercel/fun/dist/src/runtimes');
   await cpy('**/*', join(distRoot, 'runtimes'), {
     parents: true,
     cwd: runtimes,