aseprite.skia: Override the gn version to fix the build

With #83290 merged the build would fail during the configuration phase:
```
configuring
ERROR at //gn/BUILDCONFIG.gn:85:14: Script returned non-zero exit code.
  is_clang = exec_script("gn/is_clang.py",
             ^----------
Current dir: /build/source/out/Release/
Command: python /build/source/gn/gn/is_clang.py cc c++
Returned 2.
stderr:

python: can't open file '/build/source/gn/gn/is_clang.py': [Errno 2] No such file or directory
```