Optimize GitHub Actions npm install time by shrinking and reusing the right caches

[Copilot]

CI was spending unnecessary time restoring and saving large per-job dependency caches, then retrying full npm install runs with fixed sleeps. This change narrows npm caching to tarballs, reuses a shared cache across equivalent jobs, and lets npm handle transient fetch retries directly.

• Cache only the npm tarball store

  • Removed ./node_modules, ./docs-src/node_modules, and other workspace node_modules paths from Actions caches
  • Kept ~/.npm as the shared cache target, which is cheaper to restore/save and still speeds installs

• Deduplicate cache keys across equivalent jobs

  • Replaced many job-specific root dependency cache keys with one shared key:
    • Linux/Windows jobs now reuse the same root dependency cache shape per OS
    • Cache invalidation now tracks package.json and .npmrc changes

• Replace shell retry chains with npm-native fetch retries

  • Removed repeated patterns like npm install || (sleep 15 && npm install) ...
  • Added workflow-level npm fetch retry settings and prefer-offline so retries happen at the network fetch layer instead of rerunning the whole install

• Preserve non-npm caching where needed

  • Split the FoundationDB workflow cache so Docker layer caching remains separate from npm dependency caching

env:
  NPM_CONFIG_FETCH_RETRIES: 3
  NPM_CONFIG_FETCH_RETRY_FACTOR: 2
  NPM_CONFIG_FETCH_RETRY_MINTIMEOUT: 5000
  NPM_CONFIG_FETCH_RETRY_MAXTIMEOUT: 20000
  NPM_CONFIG_PREFER_OFFLINE: true

- name: Reuse npm cache folder
  uses: actions/cache@v5
  with:
    path: |
      ~/.npm
    key: ${{ runner.os }}-npm-root-v1-${{ hashFiles('**/package.json', '**/.npmrc') }}

- name: install npm dependencies
  run: npm install