git_cl: run gn.py instead of gn

fix for https://ci.chromium.org/ui/p/devtools-frontend/builders/try/dtf_presubmit_linux/b8715095169268935393/overview

Change-Id: I1fa4565ffe44ea0ed29b4b8710ddf933d19f9f44
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6536211
Commit-Queue: Brian Ryner <bryner@google.com>
Reviewed-by: Brian Ryner <bryner@google.com>
Auto-Submit: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Liviu Rau <liviurau@chromium.org>

git_cl.py

@@ -6938,7 +6938,7 @@ def _RunYapf(opts, paths, top_dir, upstream_commit):
 
 
 def _RunGnFormat(opts, paths, top_dir, upstream_commit):
-    cmd = ['gn', 'format']
+    cmd = [sys.executable, os.path.join(DEPOT_TOOLS, 'gn.py'), 'format']
     if opts.dry_run or opts.diff:
         cmd.append('--dry-run')
     return_value = 0