[snap] enable ccache in Linux Snap workflow

Also adding `push` to triggering events to populate the cache.

Signed-off-by: Michał Sawicz <michal@sawicz.net>

.github/workflows/linux-snap.patch

@@ -0,0 +1,44 @@
+--- a/snap/snapcraft.yaml
++++ b/snap/snapcraft.yaml
+@@ -32,6 +32,7 @@ parts:
+   googlemaps:
+     source: git://github.com/Subsurface/googlemaps.git
+     build-packages:
++    - ccache
+     - wget
+     override-pull: |
+       snapcraftctl pull
+@@ -75,6 +76,8 @@ parts:
+     plugin: qmake
+     options:
+     - INCLUDEPATH+=QtHeaders
++    - QMAKE_CC=ccache gcc
++    - QMAKE_CXX=ccache g++
+ 
+   desktop-qt5:
+     source: https://github.com/ubuntu/snapcraft-desktop-helpers.git
+@@ -105,7 +108,11 @@ parts:
+     source: .
+     source-type: git
+     source-subdir: libdivecomputer
++    configflags:
++    - CC=ccache gcc
++    - CXX=ccache g++
+     build-packages:
++    - ccache
+     - libbluetooth-dev
+     - libhidapi-dev
+     - libusb-dev
+@@ -127,9 +134,12 @@ parts:
+     - -DLIBGIT2_DYNAMIC=ON
+     - -DFTDISUPPORT=ON
+     - -DLIBDIVECOMPUTER_LIBRARIES=../../../stage/lib/libdivecomputer.so
++    - -DCMAKE_C_COMPILER_LAUNCHER=ccache
++    - -DCMAKE_CXX_COMPILER_LAUNCHER=ccache
+     source-type: git
+     build-packages:
+     - build-essential
++    - ccache
+     - libcurl4-gnutls-dev
+     - libftdi1-dev
+     - libgit2-dev

.github/workflows/linux-snap.yml

@@ -28,11 +28,36 @@ jobs:
         # Needed for version determination to work
         fetch-depth: 0
 
+    - name: Set up CCache
+      id: setup-ccache
+      run: |
+        sudo apt-get install ccache
+        ccache --max-size=2G
+        sg lxd -c '/snap/bin/lxc profile device add default ccache disk source=${HOME}/.ccache/ path=/root/.ccache'
+
+        # Patch snapcraft.yaml to enable ccache
+        patch -p1 < .github/workflows/linux-snap.patch
+
+        # Find common base between master and HEAD to use as cache key.
+        git -c protocol.version=2 fetch --no-tags --prune --progress --no-recurse-submodules origin master
+        echo "::set-output name=key::$( git merge-base origin/master ${{ github.sha }} )"
+
+    - name: CCache
+      uses: actions/cache@v2
+      with:
+        key: ccache-${{ runner.os }}-${{ steps.setup-ccache.outputs.key }}
+        restore-keys: |
+          ccache-${{ runner.os }}-
+        path: ~/.ccache/**
+
     - name: Build
       run: |
         # Build the `subsurface` part.
         sg lxd -c '/snap/bin/snapcraft build --use-lxd subsurface'
 
+    - name: Clear CCache stats
+      run: ccache --show-stats --zero-stats
+
     - name: Build and verify the snap
       id: build-snap
       env: