ci: use user-writable Rust paths on self-hosted runners

[andybrown668]

Summary

• Redirect RUSTUP_HOME and CARGO_HOME to user-writable paths on self-hosted lab runners before rustup runs.
• Fixes CI failures where rustup toolchain install cannot write to root-owned /usr/local/rustup/tmp.

Test plan

• Reproduce failure locally with default /usr/local/{rustup,cargo} paths
• Verify rustup toolchain install stable succeeds with user-writable paths
• Confirm Continuous Integration passes on self-hosted lab runners

Verification commands

export RUSTUP_HOME="$HOME/.rustup" CARGO_HOME="$HOME/.cargo"
mkdir -p "$RUSTUP_HOME" "$CARGO_HOME/bin"
rustup toolchain install stable --component rustfmt --component clippy --profile minimal --no-self-update

Made with Cursor

[Copilot]

Pull request overview

This PR addresses CI failures on self-hosted “lab” runners by ensuring Rust tooling uses user-writable directories for RUSTUP_HOME and CARGO_HOME before any rustup-driven toolchain installation occurs.

Changes:

• Introduces a reusable composite action (.github/actions/rust-paths) that sets RUSTUP_HOME, CARGO_HOME, and updates PATH on self-hosted runners.
• Wires the new action into the main CI workflow in jobs that invoke dtolnay/rust-toolchain.
• Integrates the same path configuration into the shared setup composite action so downstream jobs inherit the fix.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
.github/workflows/ci.yml	Adds the new rust-path configuration step before Rust toolchain setup in relevant jobs on self-hosted runners.
.github/actions/setup/action.yml	Ensures the shared environment setup configures user-writable Rust paths before installing the toolchain.
.github/actions/rust-paths/action.yml	Adds a composite action that redirects Rust tooling state to $HOME on self-hosted runners via GITHUB_ENV/GITHUB_PATH.