Move BC check to separate workflow; run against PR base

.github/workflows/backwards-compatibility.yml

@@ -0,0 +1,21 @@
+name: "Backwards compatibility"
+
+on:
+    pull_request:
+
+jobs:
+    bc-check:
+        name: "Backwards compatibility check"
+
+        runs-on: "ubuntu-latest"
+
+        steps:
+            - name: "Checkout"
+              uses: "actions/[email protected]"
+              with:
+                  fetch-depth: 0
+
+            - name: "BC Check"
+              uses: docker://nyholm/roave-bc-check-ga
+              with:
+                  args: --from=${{ github.event.pull_request.base.sha }}

.github/workflows/tests.yml

@@ -102,14 +102,6 @@ jobs:
 
             - run: vendor/bin/psalm --no-progress --stats --threads=$(nproc) --output-format=github --shepherd
 
-    bc_check:
-        name: Backward Compatibility
-        runs-on: ubuntu-latest
-        steps:
-            - uses: actions/checkout@v2
-            - name: Roave BC Check
-              uses: docker://nyholm/roave-bc-check-ga
-
     docs-lint:
         name: Markdownlint
         runs-on: ubuntu-latest