CI: CCache for macOS (#640)

Add environment options to make CCache work on macOS.

.github/workflows/macos.yml

@@ -57,6 +57,12 @@ jobs:
           ccache-macos-appleclang-
     - name: build ImpactX
       run: |
+        export CCACHE_COMPRESS=1
+        export CCACHE_COMPRESSLEVEL=10
+        export CCACHE_MAXSIZE=100M
+        export CCACHE_SLOPPINESS=time_macros
+        ccache -z
+
         source py-venv/bin/activate
 
         cmake -S . -B build            \
@@ -65,6 +71,9 @@ jobs:
           -DPython_EXECUTABLE=$(which python3)
         cmake --build build -j 3
 
+        du -hs ~/Library/Caches/ccache
+        ccache -s
+
     - name: run tests
       run: |
         source py-venv/bin/activate