ci: move semver-checks to its own job

This is necessary because it needs to run only on pull_request,
and not on push events.

.github/workflows/rust.yml

@@ -303,35 +303,3 @@ jobs:
         with:
           args: "--only-codegen"
 
-  API:
-    needs: Prepare
-    name: API - nightly toolchain
-    runs-on: ubuntu-latest
-    strategy:
-      fail-fast: false
-    steps:
-      - name: "Checkout repo"
-        uses: actions/checkout@v4
-        with:
-          fetch-depth: 0 # we need full history for cargo semver-checks
-      - name: "Select toolchain"
-        uses: dtolnay/rust-toolchain@v1
-        with:
-          toolchain: ${{ needs.Prepare.outputs.nightly_version }}
-      - name: "Install cargo-binstall"
-        uses: cargo-bins/cargo-binstall@main
-      - name: "Binstall cargo-semver-checks"
-        run: cargo binstall cargo-semver-checks --no-confirm
-      - name: "Run semver checker script"
-        run: ./contrib/check-semver.sh
-      - name: "Add PR label to breaking changes"
-        uses: actions-ecosystem/action-add-labels@v1
-        if: ${{ hashFiles('semver-break') != '' }}
-        with:
-          labels: "API break"
-      - name: Comment PR
-        uses: thollander/actions-comment-pull-request@v2
-        if: ${{ hashFiles('semver-break') != '' }}
-        with:
-          message: |
-            :rotating_light: API BREAKING CHANGE DETECTED

.github/workflows/semver-checks.yml

@@ -0,0 +1,39 @@
+on:   # yamllint disable-line rule:truthy
+  pull_request:
+
+name: Check semver breaks
+
+jobs:
+  API:
+    name: API - nightly toolchain
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+    steps:
+      - name: "Checkout repo"
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0 # we need full history for cargo semver-checks
+      - name: "Install Rustup"
+        uses: dtolnay/rust-toolchain@nightly
+      - name: "Select nightly-version"
+        run: |
+          rustup default $(cat nightly-version)
+      - name: "Install cargo-binstall"
+        uses: cargo-bins/cargo-binstall@main
+      - name: "Binstall cargo-semver-checks"
+        run: cargo binstall cargo-semver-checks --no-confirm
+      - name: "Run semver checker script"
+        run: ./contrib/check-semver.sh
+      - name: "Add PR label to breaking changes"
+        uses: actions-ecosystem/action-add-labels@v1
+        if: ${{ hashFiles('semver-break') != '' }}
+        with:
+          labels: "API break"
+      - name: Comment PR
+        uses: thollander/actions-comment-pull-request@v2
+        if: ${{ hashFiles('semver-break') != '' }}
+        with:
+          message: |
+            :rotating_light: API BREAKING CHANGE DETECTED
+