Use actions/cache@v2.

Supposed to have some speed improvements (and new features, but we don't use
those at the moment).

Author: thomasvl

.github/workflows/build.yml

@@ -59,7 +59,7 @@ jobs:
       run: make test-runtime
     - name: Cache protobuf
       id: cache-protobuf
-      uses: actions/cache@v1
+      uses: actions/cache@v2
       with:
         path: protobuf
         # NOTE: for refs that can float like 'master' the cache might be out of date!

.github/workflows/regular_conformance.yml

@@ -55,7 +55,7 @@ jobs:
         esac
     - name: Cache protobuf
       id: cache-protobuf
-      uses: actions/cache@v1
+      uses: actions/cache@v2
       with:
         path: protobuf
         # NOTE: for refs that can float like 'master' the cache might be out of date!