Cache build dependencies in GitHub Actions

[dtinth]

Background

Right now GitHub Actions doesn’t have caching. Install dependencies take 50 seconds.

Task

• Please add caching to GitHub Actions workflow file.
• The folder to cache is common/temp/pnpm-store (see: microsoft/rushstack#1847 (comment))

Hacktoberfest

If you would like to work on this task, please write a comment stating your intent. We will then assign the issue to you. To ensure continuity, the issue will be unassigned after 3 days of inactivity — so please keep us updated.