Tidy up Pkger install.

Author: chrisgavin

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
         with:
           go-version: 1.14.7
       - name: Install Pkger
-        run: go get github.com/markbates/pkger/cmd/pkger
+        run: go install --mod=readonly github.com/markbates/pkger/cmd/pkger && git diff-index --quiet --cached HEAD --
       - name: Build
         uses: goreleaser/goreleaser-action@v2
         with: