Github Actions: ccache updates

+ Specify the ccache cache directory to avoid the defaults, because
  ccache >= 4.0 puts the cache directory in system-dependent paths
+ Adjust sloppiness to increase cache hit ratio
+ print compression stats but don't err if `-x` flag is missing
  (only available only on ccache >= 4.0)
+ Print ccache config for debugging purposes
+ Enable ccache compression and limit size

Regarding the last point, ccache < v4.0 optionally compresses using the
zlib algorithm, but ccache >= v4.0 uses zstd and compression is enabled
by default.  By forcing compression to true, all platforms will need
similar size cache. We have to limit it because the default (5GB) fills
up the cache artifacts size that Github Actions offers.

Pick-to: 6.0
Change-Id: Iea72d08b223c611ee4a4443625647a1f01809b08
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

.github/workflows/ninja-build.yml

@@ -64,7 +64,7 @@ jobs:
       id: ccache
       uses: actions/cache@v2
       with:
-        path: ~/.ccache
+        path: ${{ runner.temp }}/ccache
         # "github.run_id" is unique, which causes the cache to always get
         # saved at the end of a successful run.
         key:  ccache-${{ matrix.os }}-${{ github.ref }}-${{ github.run_id }}
@@ -81,6 +81,12 @@ jobs:
       if: matrix.deps != ''
     - name: install compiler tools
       run:  ${{ matrix.install_cmd }}  ${{ matrix.tools }}  ${{ matrix.install_cmd_postfix }}
+    - name: configure ccache
+      run: |
+        ccache --set-config sloppiness=file_macro,time_macros
+        ccache --set-config cache_dir='${{ runner.temp }}'/ccache
+        ccache --set-config compression=true
+        ccache --set-config max_size=1G
     - name: print versions and environment
       run: |
         gcc --version    | head -1
@@ -88,6 +94,7 @@ jobs:
         echo Ninja `ninja --version`
         { ninja --help || true ; } 2>&1 | grep "run N jobs in parallel"
         ccache --version | head -1
+        ccache --show-config  ||  echo 'Old ccache version does not support --show-config'
         echo Environment:
         printenv
 
@@ -105,4 +112,7 @@ jobs:
 
     - name: various stats
       # Print ccache utilization statistics, then reset them.
-      run:  ccache -s && ccache -z
+      run: |
+        ccache -s
+        ccache -x 2>/dev/null || true
+        ccache -z