revert back to 0.22.0 until tarpaulin action is fixed: actions-rs/tar…

…paulin#23

It seems like the download is broken in the `actions-rs/tarpaulin` GitHub Action for all tarpaulin versions above `0.22.0`. See this pending fix for details:
actions-rs/tarpaulin#23

.github/workflows/rust.yml

@@ -67,7 +67,7 @@ jobs:
       - name: Run cargo-tarpaulin
         uses: actions-rs/[email protected]
         with:
-          version: "0.26.0"
+          version: "0.22.0"
           args: "--all-features --doc --examples --out Lcov -- --test-threads 1"
 
       - name: Upload coverage