ci: cache fairmq compilation with ccache

FairMQ's own sources (library, examples, tests) were recompiled from scratch
in every matrix job on every push.

- add hendrikmuhs/ccache-action to build, sanitizers and static-analysis jobs
- set CMAKE_C/CXX_COMPILER_LAUNCHER=ccache so cmake routes through it
- key the cache per (job, env, gcc) since ccache hashes the compiler

.github/workflows/ci.yml

@@ -36,6 +36,12 @@ jobs:
           gcc: ${{ matrix.gcc }}
           env: ${{ matrix.env }}
 
+      - name: ccache
+        uses: hendrikmuhs/ccache-action@v1
+        with:
+          key: ${{ github.job }}-${{ matrix.env }}-gcc${{ matrix.gcc }}
+          max-size: 500M
+
       - name: Configure and Build
         uses: threeal/cmake-action@v2
         with:
@@ -44,6 +50,8 @@ jobs:
             CMAKE_BUILD_TYPE=RelWithDebInfo
             CMAKE_INSTALL_PREFIX=${{ github.workspace }}/install
             BUILD_TESTING=ON
+            CMAKE_C_COMPILER_LAUNCHER=ccache
+            CMAKE_CXX_COMPILER_LAUNCHER=ccache
 
       - name: Test
         uses: threeal/ctest-action@v1
@@ -83,6 +91,12 @@ jobs:
         with:
           gcc: '14'
 
+      - name: ccache
+        uses: hendrikmuhs/ccache-action@v1
+        with:
+          key: ${{ github.job }}-${{ matrix.sanitizer.name }}
+          max-size: 500M
+
       - name: Configure and Build
         uses: threeal/cmake-action@v2
         with:
@@ -92,6 +106,8 @@ jobs:
           options: |
             CMAKE_BUILD_TYPE=Debug
             BUILD_TESTING=ON
+            CMAKE_C_COMPILER_LAUNCHER=ccache
+            CMAKE_CXX_COMPILER_LAUNCHER=ccache
             ${{ matrix.sanitizer.options }}
 
       - name: Test
@@ -116,6 +132,12 @@ jobs:
         with:
           gcc: '14'
 
+      - name: ccache
+        uses: hendrikmuhs/ccache-action@v1
+        with:
+          key: ${{ github.job }}
+          max-size: 500M
+
       - name: Configure and Build
         uses: threeal/cmake-action@v2
         with:
@@ -124,6 +146,8 @@ jobs:
             CMAKE_BUILD_TYPE=Debug
             BUILD_TESTING=ON
             RUN_STATIC_ANALYSIS=ON
+            CMAKE_C_COMPILER_LAUNCHER=ccache
+            CMAKE_CXX_COMPILER_LAUNCHER=ccache
 
       - name: Check for warnings
         run: |