Bump tarpaulin image to 0.28.0 in CI

Author: ramosbugs

.github/workflows/main.yml

@@ -97,7 +97,7 @@ jobs:
   coverage:
     runs-on: ubuntu-latest
     container:
-      image: xd009642/tarpaulin:0.26.0
+      image: xd009642/tarpaulin:0.28.0
       options: --security-opt seccomp=unconfined
     steps:
       - uses: actions/checkout@v2