fix(ci): use portable nightly toolchain name in setup action

The dynamic toolchain detection failed because mise sets RUSTUP_TOOLCHAIN=1.94.1, causing `rustup toolchain list | grep nightly` to return empty. Using `nightly` directly works on all platforms. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>


Author aaronmallen.dev
Co-author Claude Opus 4.6 (1M context) Date Commit Parent 42f2f16d Change ID
+2 -3
1 changed file
+2 -3
1 changed file