Split CI step between build and test

Author: vector-of-bool

.github/workflows/ci.yml

@@ -52,7 +52,14 @@ jobs:
           - use_vcpkg=true
     steps:
       - *checkout
-      - run: |
-          bash tools/earthly.sh +build \
+      - name: Build
+        run: bash tools/earthly.sh +build \
+          --from=${{matrix.from}} \
+          --${{matrix.vcpkg_arg}}
+      # This step will use the same cached +build results as long as all Earthly
+      # build arguments are equivalent. Make sure of that!
+      - name: Test
+        run: |
+          bash tools/earthly.sh -a +test/results.xml results.xml \
             --from=${{matrix.from}} \
             --${{matrix.vcpkg_arg}}