Enable use of ccache for Unix CI builds

This should speed up build step for the common case when not too many
files have to be recompiled.

.github/workflows/ci.yml

@@ -111,6 +111,11 @@ jobs:
         with:
           submodules: 'recursive'
 
+      - name: Install CCache
+        uses: hendrikmuhs/ccache-action@v1
+        with:
+          key: ${{ matrix.name }}
+
       - name: Set up build system
         run: |
           echo LD_LIBRARY_PATH=`pwd`/lib >> $GITHUB_ENV
@@ -118,6 +123,8 @@ jobs:
           wxPROC_COUNT=`./build/tools/proc_count.sh`
           echo wxBUILD_ARGS=-j$wxPROC_COUNT >> $GITHUB_ENV
 
+          echo "PATH=/usr/lib/ccache:$PATH" >> $GITHUB_ENV
+
           # Setting this variable suppresses "Error retrieving accessibility bus address"
           # messages from WebKit tests that we're not interested in.
           echo NO_AT_BRIDGE=1 >> $GITHUB_ENV