ci: [refactor] Allow overwriting check option in run helper

Also, use str(e) consistently in all run helpers.

This refactor does not change any behavior.

This can be reviewed by checking that all instances are exactly
identical code now:
$ git grep --function-context 'def run(cmd'

.github/ci-test-each-commit-exec.py

@@ -10,10 +10,11 @@ import shlex
 
 def run(cmd, **kwargs):
     print("+ " + shlex.join(cmd), flush=True)
+    kwargs.setdefault("check", True)
     try:
-        return subprocess.run(cmd, check=True, **kwargs)
+        return subprocess.run(cmd, **kwargs)
     except Exception as e:
-        sys.exit(e)
+        sys.exit(str(e))
 
 
 def main():

ci/test/02_run_container.py

@@ -17,7 +17,7 @@ def run(cmd, **kwargs):
     try:
         return subprocess.run(cmd, **kwargs)
     except Exception as e:
-        sys.exit(e)
+        sys.exit(str(e))
 
 
 def main():