rust-beta job compiles the pinned toolchain, not beta (same mechanism as #137)

[plusky]

Summary

rust-beta has been re-testing the pinned toolchain, not beta. It installs
beta via the action's toolchain: beta input, then compiles with 1.98.0,
because rust-toolchain.toml outranks the rustup default that input sets
— the same mechanism as #137, which fixed only rust-msrv.

Evidence

The pinned dtolnay/rust-toolchain action (2c7215f) never exports
RUSTUP_TOOLCHAIN; its only selection step is

- run: rustup default ${{steps.parse.outputs.toolchain}}
  continue-on-error: true

and precedence is RUSTUP_TOOLCHAIN > toolchain file > rustup default,
reproduced locally: with rustup default 1.88 set exactly as the action
leaves it, a directory pinning 1.98.0 in rust-toolchain.toml reports
rustc 1.98.0, while the same shell with RUSTUP_TOOLCHAIN=1.88 exported
reports rustc 1.88.0.

rust-msrv now exports RUSTUP_TOOLCHAIN and asserts the effective
version (#137). rust-beta does neither, so its purpose — early warning
about the upcoming compiler — is not being served.

Why this is a judgement call, not a trivial fix

The one-line change is a RUSTUP_TOOLCHAIN: beta job env. Turning it on is
safe for gating: the job is continue-on-error: true and is not among
main's required contexts. But it will surface real new-lint noise for the
first time, which is the point of the job and also the reason nobody has
noticed it was inert.

Suggested direction

• Export RUSTUP_TOOLCHAIN: beta for the job, and assert the effective
  toolchain is a beta build so it cannot silently regress to the pin —
  mirroring the assert rust-msrv job does not test the MSRV: rust-toolchain.toml outranks its toolchain input #137 added to rust-msrv.
• Expect and triage the first run's lint output rather than muting it.

Acceptance criteria

• rust-beta demonstrably compiles with a beta rustc, asserted in the
  job rather than implied by an input.
• The job stays non-gating (continue-on-error, not a required check).

Provenance: found while fixing #137; out of scope for that PR.