CI: Use common linting workflow

After #12107 it's much easier for black, isort and flake8 to find the
scripts we want them to lint.

docs/code_style.md

@@ -24,7 +24,7 @@ pip install -e ".[lint,mypy]"
     functionality) with:
 
     ```sh
-    black . --exclude="\.tox|build|env"
+    black .
     ```
 
 -   **flake8**
@@ -35,7 +35,7 @@ pip install -e ".[lint,mypy]"
     Check all application and test code with:
 
     ```sh
-    flake8 synapse tests
+    flake8 .
     ```
 
 -   **isort**
@@ -46,11 +46,9 @@ pip install -e ".[lint,mypy]"
     Auto-fix imports with:
 
     ```sh
-    isort -rc synapse tests
+    isort .
     ```
 
-    `-rc` means to recursively search the given directories.
-
 It's worth noting that modern IDEs and text editors can run these tools
 automatically on save. It may be worth looking into whether this
 functionality is supported in your editor for a more convenient