fix(ci): improve ccache persistence across builds

Use github.run_id for cache save keys so every build saves its
accumulated ccache objects, instead of the previous date-based keys
which froze the cache after the first save each day (cache-hit=true
prevented re-saving).

Restore keys still use branch/main prefixes for warm fallback.

Also add ccache-build-main- fallback to release workflow so release
builds can bootstrap from the latest main build cache.

.gitea/workflows/build.yml

@@ -68,16 +68,12 @@ jobs:
           export PATH="$HOME/.pixi/bin:$PATH"
           pixi --version
 
-      - name: Compute cache date key
-        id: cache-date
-        run: echo "date=$(date -u +%Y%m%d)" >> $GITHUB_OUTPUT
-
       - name: Restore ccache
         id: ccache-restore
         uses: https://git.kindred-systems.com/actions/cache.git/restore@v4
         with:
           path: /tmp/ccache-kindred-create
-          key: ccache-build-${{ github.ref_name }}-${{ steps.cache-date.outputs.date }}
+          key: ccache-build-${{ github.ref_name }}-${{ github.run_id }}
           restore-keys: |
             ccache-build-${{ github.ref_name }}-
             ccache-build-main-
@@ -98,11 +94,11 @@ jobs:
         run: pixi run ccache -s
 
       - name: Save ccache
-        if: always() && steps.ccache-restore.outputs.cache-hit != 'true'
+        if: always()
         uses: https://git.kindred-systems.com/actions/cache.git/save@v4
         with:
           path: /tmp/ccache-kindred-create
-          key: ccache-build-${{ github.ref_name }}-${{ steps.cache-date.outputs.date }}
+          key: ccache-build-${{ github.ref_name }}-${{ github.run_id }}
 
       - name: Run C++ unit tests
         continue-on-error: true

.gitea/workflows/release.yml

@@ -78,18 +78,15 @@ jobs:
           export PATH="$HOME/.pixi/bin:$PATH"
           pixi --version
 
-      - name: Compute cache date key
-        id: cache-date
-        run: echo "date=$(date -u +%Y%m%d)" >> $GITHUB_OUTPUT
-
       - name: Restore ccache
         id: ccache-restore
         uses: https://git.kindred-systems.com/actions/cache.git/restore@v4
         with:
           path: /tmp/ccache-kindred-create
-          key: ccache-release-linux-${{ steps.cache-date.outputs.date }}
+          key: ccache-release-linux-${{ github.run_id }}
           restore-keys: |
             ccache-release-linux-
+            ccache-build-main-
 
       - name: Prepare ccache
         run: |
@@ -109,11 +106,11 @@ jobs:
         run: pixi run ccache -s
 
       - name: Save ccache
-        if: always() && steps.ccache-restore.outputs.cache-hit != 'true'
+        if: always()
         uses: https://git.kindred-systems.com/actions/cache.git/save@v4
         with:
           path: /tmp/ccache-kindred-create
-          key: ccache-release-linux-${{ steps.cache-date.outputs.date }}
+          key: ccache-release-linux-${{ github.run_id }}
 
       - name: Clean up intermediate build files
         run: |