CI: cache luarocks folder (#1057)

* ci: cache luarocks folder

* ci: make cache only save if it's not present

As per
actions/toolkit#658 (comment),
overwriting a saved cache doesn't work and throws a warning.

Author: santimorph

.github/workflows/ci.yml

@@ -28,6 +28,13 @@ jobs:
 
     - uses: luarocks/gh-actions-luarocks@master
 
+    - name: Restore cached LuaRocks packages
+      id: cache
+      uses: actions/cache/restore@v4
+      with:
+        path: ${{ runner.os == 'Windows' && 'C:\\Users\\runneradmin\\AppData\\Roaming\\luarocks' || format('{0}/.luarocks', github.workspace) }}
+        key: ${{ runner.os }}-luarocks-${{ matrix.lua-version }}-${{ hashFiles('**/luarocks.lock') }}
+
     - name: Build
       run: |
         luarocks make --only-deps
@@ -37,3 +44,10 @@ jobs:
         luarocks lint tl-dev-1.rockspec
         make selfbuild
         luarocks test
+
+    - name: Save LuaRocks packages
+      if: steps.cache.outputs.cache-hit != 'true'
+      uses: actions/cache/save@v4
+      with:
+        path: ${{ runner.os == 'Windows' && 'C:\\Users\\runneradmin\\AppData\\Roaming\\luarocks' || format('{0}/.luarocks', github.workspace) }}
+        key: ${{ runner.os }}-luarocks-${{ matrix.lua-version }}-${{ hashFiles('**/luarocks.lock') }}