Update CI build environment for macOS mpy-cross build

CI is now warning us:
```
[Build CI: .github#L1](https://github.com/adafruit/circuitpython/pull/6619/files#annotation_4061653702)
The macOS-10.15 environment is deprecated, consider switching to macos-11(macos-latest), macos-12 instead. For more details see https://github.com/actions/virtual-environments/issues/5583
```