Revert complex `corepack` GitHub Actions changes

[karlhorky]

Now that the Failed to switch pnpm to v9.8.0. Looks like pnpm CLI is missing at errors have seemingly been addressed in pnpm (see PR and issue below)...

• fix: self-update should not read pnpm settings from current package.json pnpm/pnpm#9196
• Looks like pnpm CLI is missing at \pnpm\.tools\ pnpm/pnpm#9192 (comment)

...we can try removing the switch to Corepack and the related workarounds introduced in the last commits / PR:

• 3fe3d81...e80220b
• Fix Corepack integration with GitHub Actions workflow and test #631