fix: set RUSTUP_TOOLCHAIN=stable for MSRV check in publish workflow

[mmagician]

Summary

• The publish workflow's MSRV check (check-msrv.sh) fails for every workspace member because rust-toolchain.toml pins the channel to 1.91, which interferes with cargo-msrv's toolchain resolution
• The nightly workflow already avoids this by setting RUSTUP_TOOLCHAIN: stable (with an explicit comment explaining why)
• This applies the same fix to the publish workflow's MSRV check step

Context

Failed run: https://github.com/0xMiden/node/actions/runs/23306258363/job/67781096134

The nightly workflow's MSRV checks all pass because it sets env: RUSTUP_TOOLCHAIN: stable. The publish workflow was missing this override, causing cargo msrv verify to fail when rust-toolchain.toml is present.

Test plan

• Re-run the release workflow after merging; the MSRV check step should pass

🤖 Generated with Claude Code

[Mirko-von-Leipzig]

Ah thanks! I didn't know msrv was also present here.

[mmagician]

hmm judging by the failed jobs here, this might not be needed: the issue was in that the newly added miden-genesis binary still specified alpha.1 of protocol crates, while all the others specified alpha.2 (both with exact version match requirement).
I fixed this and the MSVR check passed - closing this now.