ci: only save cache on main

Author: Firestar99

.github/workflows/ci.yaml

@@ -45,6 +45,7 @@ jobs:
         with:
           add-job-id-key: "false"
           shared-key: "base"
+          save-if: ${{ github.ref_name == 'main' }}
       # Fetch dependencies in a separate step to clearly show how long each part
       # of the testing takes
       - name: cargo fetch --locked