Enable flake8-errmsg linting rules (#267)

See https://docs.astral.sh/ruff/rules/#flake8-errmsg-em.

src/common/git.py

@@ -20,7 +20,8 @@ class Git:
             child = run(f"git {cmd}", capture_output=True, timeout=300, check=True, shell=True, cwd=self.repo_dir)
             return child.stdout.decode("utf-8").strip().split("\n")
         except ChildProcessError as ex:
-            raise RuntimeError(f"Failed to run '{cmd}': {ex}") from ex
+            msg = f"Failed to run '{cmd}': {ex}"
+            raise RuntimeError(msg) from ex
 
     def setup(self, bare: bool = False) -> None:
         """Creates the repository path and runs: