gha: change ccache key for redpanda workflow

Use git ref as part of the key and remove timestamp. The current limit
from Github is 5GB for all cached files across a repository. With this
change, there is only one ccache file uploaded per branch, instead of
one for each distinct build. This results in having one for the main
branch, as well as one per PR.

Signed-off-by: Ivo Jimenez <[email protected]>

.github/workflows/redpanda-build.yml

@@ -31,23 +31,13 @@ jobs:
               .
             rm .dockerignore
 
-        # there is no analog to ${{ github.workspace }} for the home directory
-        - name: prepare cache variables
-          id: prepare_cache_variables
-          run: |
-            echo ::set-output name=home::$HOME
-            echo ::set-output name=timestamp::$(date +'%Y-%m-%d-%H-%M-%S')
-
         - name: ccache cache files
           uses: actions/cache@v2
           with:
-            path: ${{ steps.prepare_cache_variables.outputs.home }}/.ccache
-            key: ${{ matrix.container }}-ccache-${{ steps.prepare_cache_variables.outputs.timestamp }}
-            restore-keys: ${{ matrix.container }}-ccache-
+            path: /dev/shm/redpanda
+            key: ccache-${{github.ref}}
+            restore-keys: ccache
 
-        # at the time of this writing the ccache for a full build is about
-        # 70MB (due to the compression) but it may grow if there are many
-        # commits with changes within 1 branch and thus the 200MB upper limit
         - name: build & test
           run: |
             export CCACHE_COMPRESS=true