Use direct path to go.sum

The `hashFiles("**/go.sum*)` approach walks the entire direectory
tree, including after tests are run.  If there are broken symlinks,
it can cause the post-cache to fail.

Author: bbockelm

.github/workflows/test-linux.yml

@@ -58,7 +58,7 @@ jobs:
         path: |
           ~/.cache/go-build
           ~/go/pkg/mod
-        key: ${{ runner.os }}-go-${{ hashFiles('**/go.sum') }}
+        key: ${{ runner.os }}-go-${{ hashFiles('go.sum') }}
         restore-keys: |
           ${{ runner.os }}-go-
     - name: Install gotestsum