Refactor nix configuration on ci and the aarch64 gcc dependency on x86_64 machines

[potsrevennil]

1. Utilization of pkgCross.aarch-multiplatform:

• The pkgCross.aarch-multiplatform package in Nix is now used to set up GCC targeting the AArch64 platform. This change enhances clarity and simplicity.
• The cumbersome arm-gnu-gcc.nix file can thus be removed.
• Note: AArch64 macOS will continue using the native Clang compiler as per the original setup.

2. Cross Prefix Modification:

• The cross prefix for GCC in pkgCross.aarch-multiplatform has changed from aarch64-none-linux-gnu- to aarch64-unknown-linux-gnu-.
• To accommodate this, the test script has been updated to allow users to specify the cross prefix they are using.

3. CI Configuration Refactor:

• The CI configuration has been streamlined by utilizing GitHub defaults for setting up the Nix shell, thereby avoiding multiple shell declarations.

[mkannwischer]

I tested that I can cross compile on x86 and the produced binaries still run on A55. Functionally this is looking good for me. If you fix the one comment I had, we can get this merged.
Thanks @potsrevennil!

I can't test cross compilation since I only have aarch64 machines

[mkannwischer]

Thanks @potsrevennil! This is much cleaner now!

I tested that everything still works on my laptop (cross-compiled + qemu) and on my RPi (natively).