ci: Only save caches on main

This restores caches on all branches and pull requests, but only stores them on the main
branch. This prevents those caches from being evicted early when we hit the 10 GB limit
quickly.

.github/workflows/build.yaml

@@ -114,8 +114,9 @@ jobs:
     runs-on: ${{ matrix.runs-on }}
     steps:
       - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
-      - name: Stack working files cache
-        uses: actions/cache@ab5e6d0c87105b4c9c2047343972218f562e4319 # v4.0.1.0.1
+      - name: Restore stack working files cache
+        uses: actions/cache/restore@ab5e6d0c87105b4c9c2047343972218f562e4319 # v4.0.1
+        id: restore-cache
         with:
           path: ${{ matrix.cache }}
           key: stack-${{ runner.os }}-${{ hashFiles('stack.yaml.lock') }}
@@ -137,6 +138,12 @@ jobs:
             result/postgrest
             result/postgrest.exe
           if-no-files-found: error
+      - name: Save stack working files cache
+        if: ${{ github.ref == 'refs/heads/main' }}
+        uses: actions/cache/save@v4
+        with:
+          path: ${{ matrix.cache }}
+          key: ${{ steps.restore-cache.outputs.cache-primary-key }}
 
 
   freebsd:
@@ -170,8 +177,9 @@ jobs:
       - name: Fix caching
         run: |
           mkdir ~/.cabal
-      - name: Cache
-        uses: actions/cache@ab5e6d0c87105b4c9c2047343972218f562e4319 # v4.0.1.0.1
+      - name: Restore cache
+        uses: actions/cache/restore@ab5e6d0c87105b4c9c2047343972218f562e4319 # v4.0.1
+        id: restore-cache
         with:
           path: |
             ~/.cabal/packages
@@ -186,3 +194,12 @@ jobs:
           cabal build --only-dependencies --enable-tests --enable-benchmarks
       - name: Build
         run: cabal build --enable-tests --enable-benchmarks all
+      - name: Save cache
+        if: ${{ github.ref == 'refs/heads/main' }}
+        uses: actions/cache/save@v4
+        with:
+          path: |
+            ~/.cabal/packages
+            ~/.cabal/store
+            dist-newstyle
+          key: ${{ steps.restore-cache.outputs.cache-primary-key }}