GitHub Actions: Add flake8 problem matcher (#37465)

.github/workflows/ci.yaml

@@ -308,6 +308,9 @@ jobs:
         run: |
           echo "Failed to restore Python virtual environment from cache"
           exit 1
+      - name: Register flake8 problem matcher
+        run: |
+          echo "::add-matcher::.github/workflows/matchers/flake8.json"
       - name: Run flake8
         run: |
           . venv/bin/activate

.github/workflows/matchers/flake8.json

@@ -0,0 +1,30 @@
+{
+  "problemMatcher": [
+    {
+      "owner": "flake8-error",
+      "severity": "error",
+      "pattern": [
+        {
+          "regexp": "^(.*):(\\d+):(\\d+):\\s(E\\d{3}\\s.*)$",
+          "file": 1,
+          "line": 2,
+          "column": 3,
+          "message": 4
+        }
+      ]
+    },
+    {
+      "owner": "flake8-warning",
+      "severity": "warning",
+      "pattern": [
+        {
+          "regexp": "^(.*):(\\d+):(\\d+):\\s([CDFNW]\\d{3}\\s.*)$",
+          "file": 1,
+          "line": 2,
+          "column": 3,
+          "message": 4
+        }
+      ]
+    }
+  ]
+}