ci: add caching of cargo artifacts.

Lifted from the actions/cache docs[0].

[0]: https://github.com/actions/cache/blob/main/examples.md#rust---cargo

Author: cpu

.github/workflows/ci.yml

@@ -115,3 +115,17 @@ jobs:
           files: ./lcov.info
           fail_ci_if_error: false
           verbose: true
+
+  cache:
+    name: Update cache
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/cache@v3
+      with:
+        path: |
+          ~/.cargo/bin/
+          ~/.cargo/registry/index/
+          ~/.cargo/registry/cache/
+          ~/.cargo/git/db/
+          target/
+        key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}