Fix: x86_64 musl target on rust-cross action

To track this, the following issue was raised, and it would be nice to fix and contribute to future requirements:
https://github.com/houseabsolute/actions-rust-cross/issues/22

Unfortunately, we haven't solved the linking issues on the x86_64 musl target.
For now, it's available as a GNU option, but a musl target will be added using the old rust-cross actions, which work.
This allows BlueOS extensions to be available on all 3 targets (armv7, aarch64, and x86_64) running on tiny Alpine containers.