ci: pin cargo-tarpaulin to previous version

Current 0.27.3 version pulls in a dependency (clap_derive) which
requires Rust 1.74

Author: daviddrysdale

.github/workflows/ci.yml

@@ -211,6 +211,7 @@ jobs:
         with:
           crate: cargo-tarpaulin
           use-tool-cache: true
+          version: 0.27.2
       - run: cargo tarpaulin --verbose --ignore-tests --all-features --timeout=600 --out xml
       - name: Upload to codecov.io
         run: |