gfxstream: codegen: don't require clang-format

When auto-generating at build time, this causes
problems for AOSP:

https://android.googlesource.com/platform/build/+/main/Changes.md#PATH_Tools

Don't require via environment variable.

Reviewed-by: Aaron Ruby <aruby@blackberry.com>
Acked-by: Yonggang Luo <luoyonggang@gmail.com>
Acked-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27246>

src/gfxstream/codegen/scripts/cereal/common/codegen.py

@@ -187,9 +187,11 @@ class Module(object):
                 file.write(modified_content)
 
         clang_format_command = shutil.which('clang-format')
-        assert (clang_format_command is not None)
 
         def formatFile(filename: Path):
+            if "GFXSTREAM_NO_CLANG_FMT" in os.environ:
+                return
+            assert (clang_format_command is not None)
             assert (subprocess.call([clang_format_command, "-i",
                     "--style=file", str(filename.resolve())]) == 0)