workflow: remake caching to use @actions/cache and cache only main as baseline

.gitea/workflows/build.yaml

@@ -4,6 +4,9 @@
 name: 'Build system'
 
 on:
+  push:
+    branches:
+      - 'main'
   pull_request:
     branches:
       - 'main'
@@ -30,10 +33,14 @@ jobs:
         uses: actions/checkout@v4
         with:
           submodules: true
+          fetch-depth: 0
 
       - name: Get describe
         id: vars
-        run: echo "sha_short=$(git rev-parse --short HEAD)" >> $GITEA_OUTPUT
+        run: |
+          echo "sha=$(git rev-parse HEAD)" >> $GITEA_OUTPUT
+          echo "sha_short=$(git rev-parse --short HEAD)" >> $GITEA_OUTPUT
+          echo "describe=$(git describe --tags --always 2>/dev/null || git rev-parse --short HEAD)" >> $GITEA_OUTPUT
 
       - name: Get toolchain hash
         id: toolchain-hash
@@ -109,17 +116,17 @@ jobs:
           key: kolibri-toolchain-${{ steps.toolchain-hash.outputs.hash }}
 
       - name: Restore build cache
-        run: |
-          CACHE_DIR="/build-cache/${{ steps.toolchain-hash.outputs.hash }}"
-          if [ -d "$CACHE_DIR/.tup" ]; then
-            cp -a "$CACHE_DIR/.tup" .tup
-            cp -a "$CACHE_DIR/build-en_US" build-en_US 2>/dev/null || true
-            cp -a "$CACHE_DIR/build-ru_RU" build-ru_RU 2>/dev/null || true
-            cp -a "$CACHE_DIR/build-es_ES" build-es_ES 2>/dev/null || true
-            echo "Restored build cache from $CACHE_DIR"
-          else
-            echo "No build cache found — first build"
-          fi
+        id: cache-build
+        uses: actions/cache/restore@v4
+        with:
+          path: |
+            .tup
+            build-en_US
+            build-ru_RU
+            build-es_ES
+          key: kolibri-build-main-${{ steps.toolchain-hash.outputs.hash }}-${{ steps.vars.outputs.sha }}
+          restore-keys: |
+            kolibri-build-main-${{ steps.toolchain-hash.outputs.hash }}-
 
       - name: Sync tup DB with filesystem ctime
         run: |
@@ -225,12 +232,12 @@ jobs:
           path: build-es_ES/data/kolibri.raw
 
       - name: Save build cache
-        run: |
-          CACHE_DIR="/build-cache/${{ steps.toolchain-hash.outputs.hash }}"
-          rm -rf "$CACHE_DIR"
-          mkdir -p "$CACHE_DIR"
-          cp -a .tup "$CACHE_DIR/.tup"
-          cp -a build-en_US "$CACHE_DIR/build-en_US"
-          cp -a build-ru_RU "$CACHE_DIR/build-ru_RU"
-          cp -a build-es_ES "$CACHE_DIR/build-es_ES"
-          echo "Saved build cache to $CACHE_DIR ($(du -sh "$CACHE_DIR" | cut -f1))"
+        if: gitea.ref == 'refs/heads/main' && steps.cache-build.outputs.cache-hit != 'true'
+        uses: actions/cache/save@v4
+        with:
+          path: |
+            .tup
+            build-en_US
+            build-ru_RU
+            build-es_ES
+          key: kolibri-build-main-${{ steps.toolchain-hash.outputs.hash }}-${{ steps.vars.outputs.sha }}