`buildCSS` performance degradation caused by postcss 8.4.17 to 8.4.30 upgrade.

[andelf]

          Now, `buildCSS` never finishes on my local machine.

Originally posted by @andelf in #9894 (comment)

With yarn release.

UPDATE: It actually requires 1 hour:

branch: #10278

[andelf]

Caused by https://github.com/postcss/postcss 8.4.17 to 8.4.30 upgrade.

[andelf]

After some investigation, this is caused by a series of package update of postcss and it's postcss-* deps.

[andelf]

Keep this opened as buildCSS and css file size should be fixed.