CI: Cache based on the Swift image

- Instead of manually controlling part of the key, use the Swift image name.

With the Swift 5.0.x image, we've seen things not be there (zlib header), so
this should ensure the fact that configure could act differently on each Swift
image will be better handled.

Author: thomasvl

.github/workflows/build.yml

@@ -18,15 +18,6 @@ jobs:
         # tag: "ref/tags/v3.11.4"
         # branch: "ref/heads/master"
         protobuf_git: ["ref/heads/master"]
-        include:
-          - swift: "5.2.4-bionic"
-            platform: "ubuntu-18.04"
-          - swift: "5.1.5-bionic"
-            platform: "ubuntu-18.04"
-          - swift: "5.0.3-bionic"
-            platform: "ubuntu-18.04"
-          - swift: "4.2.4"
-            platform: "ubuntu-16.04"
     container:
       image: swift:${{ matrix.swift }}
     steps:
@@ -65,7 +56,7 @@ jobs:
       with:
         path: protobuf
         # NOTE: for refs that can float like 'master' the cache might be out of date!
-        key: ${{ runner.os }}-${{ matrix.platform}}-protobuf-${{ steps.get-sha.outputs.sha }}
+        key: ${{ runner.os }}-${{ matrix.swift}}-protobuf-${{ steps.get-sha.outputs.sha }}
     - name: Checkout protobuf repo
       if: steps.cache-protobuf.outputs.cache-hit != 'true'
       uses: actions/checkout@v2