update toolchain to 2025-03-03

[tones111]

This change bumps the toolchain to a recent nightly that incorporates LLVM codegen improvements and avr-none target.

Much thanks to @Patryk27 for all your hard work

[tones111]

I'm not sure why the CI build of the example isn't seeing the processor flag. cargo build works fine for me locally and I was able to run the example on an arduino board.

UPDATE:
looks like the action uses a default rustflags value and we need to explicitly tell it to use ours.
https://github.com/actions-rust-lang/setup-rust-toolchain/blob/main/README.md?plain=1#L69-L78