use hash instead of date in bootstrap cache keys

Without some distinct identifier, downloaded sources will accumulate
(and have been since the last time the datestamp was updated).

The old caches are ignored and will be purged according to GitHub's
cache expiration, because otherwise we'd continue to carry 4 years'
worth of outdated source tarballs.

Author: geekosaur

.github/workflows/bootstrap.yml

@@ -32,6 +32,8 @@ jobs:
           rm -rf ~/.config/cabal
           rm -rf ~/.cache/cabal
 
+      - uses: actions/checkout@v6
+
       # runner.os isn't sufficient for binary compatible caches
       - name: Get runner OS/version for cache keys
         id: get-osver
@@ -42,10 +44,9 @@ jobs:
         id: bootstrap-cache
         with:
           path: "/home/runner/work/cabal/cabal/_build"
-          key: bootstrap-${{ steps.get-osver.outputs.osver }}-${{ matrix.ghc }}-20221115-${{ github.sha }}
-          restore-keys: bootstrap-${{ steps.get-osver.outputs.osver }}-${{ matrix.ghc }}-20221115-
+          key: bootstrap-${{ steps.get-osver.outputs.osver }}-${{ matrix.ghc }}-${{ hashFiles(format('bootstrap/linux-{0}.json', matrix.ghc)) }}-${{ github.sha }}
+          restore-keys: bootstrap-${{ steps.get-osver.outputs.osver }}-${{ matrix.ghc }}-${{ hashFiles(format('bootstrap/linux-{0}.json', matrix.ghc)) }}-
 
-      - uses: actions/checkout@v6
       - uses: haskell-actions/setup@v2
         with:
           ghc-version: ${{ matrix.ghc }}