Fix bug where we would try to use cross on ARM hosts for all targets

If the target is ARM and the host is ARM, this should work with just `cargo`. I'm not sure if this
works for all ARM host/target combos, or if the CPU architecture must be an exact match.

But it doesn't really matter too much, since at least for now there's no ARM builds for
`cross` (https://github.com/cross-rs/cross/issues/1612). So even if cross-compilation doesn't work
natively, we couldn't use `cross` in this case either.

I also rewrote the `set-cross-compile.sh` in Python. By "I", I mean I had Claude do it and then I
tweaked it.

action.yml

@@ -76,7 +76,7 @@ runs:
     - name: Determine whether we need to cross-compile
       id: determine-cross-compile
       shell: bash
-      run: set-cross-compile.sh ${{ inputs.target }}
+      run: set-cross-compile.py ${{ inputs.target }}
     - name: Install toolchain
       uses: dtolnay/rust-toolchain@master
       with: