ci: Mark the regex to find chunks in code style checks as regex string

Fixes
    check-style.py:23: SyntaxWarning: invalid escape sequence '\+'
    check-style.py:24: SyntaxWarning: invalid escape sequence '\d'

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3454>

check-style.py

@@ -20,8 +20,8 @@ def run_diff(sha):
     return proc.stdout.strip().splitlines()
 
 def find_chunks(diff):
-    file_entry_re = re.compile('^\+\+\+ b/(.*)$')
-    diff_chunk_re = re.compile('^@@ -\d+,\d+ \+(\d+),(\d+)')
+    file_entry_re = re.compile(r'^\+\+\+ b/(.*)$')
+    diff_chunk_re = re.compile(r'^@@ -\d+,\d+ \+(\d+),(\d+)')
     file = None
     chunks = []