CI: Verify semver compatibility when tags is pushed

Add a job that checks compatibility when a tag is pushed.
The command executed is just
`cargo semver-checks -p scylla -p scylla-cql` because it's behavior is
exactly what we want. It gets current version from Cargo.toml,
calculates previous version (unless current version is on crates.io -
which should not be the case) and checks that the release type
(major / minor / patch) is correct.

.github/workflows/semver_checks.yml

@@ -7,6 +7,9 @@ on:
     branches:
     - main
     - 'branch-*'
+  push:
+    tags:
+      - v*.*.*
 
 env:
   CARGO_TERM_COLOR: always
@@ -71,3 +74,14 @@ jobs:
       env:
         GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         GH_REPO: ${{ github.repository }}
+
+  semver-push-tag:
+    runs-on: ubuntu-latest
+    if: github.event_name == 'push'
+    timeout-minutes: 30
+    steps:
+    - uses: actions/checkout@v3
+    - name: Install semver-checks
+      run: cargo install cargo-semver-checks --no-default-features
+    - name: Verify that's it's safe to publish the version.
+      run: cargo semver-checks -p scylla -p scylla-cql