Verify each finding against the current code and only fix it if needed.

Outside diff comments:
In @.github/workflows/ci.yaml:
- Around line 414-427: The workflow uses RUSTUP_TOOLCHAIN: nightly but the "Run
tests under miri" step calls cargo with a non-existent toolchain variable (+${{
env.NIGHTLY_CHANNEL }}); remove the +${{ env.NIGHTLY_CHANNEL }} prefix from the
"cargo +... miri test -p rustpython-vm -- miri_test" command so it simply runs
"cargo miri test -p rustpython-vm -- miri_test" and relies on the job-level
RUSTUP_TOOLCHAIN setting.
- Around line 360-364: The workflow step currently mixes an action usage and a
run command (the step with uses:
actions/setup-python@a309ff8b426b58ec0e45f0f869d46889d02405 and the run: rustup
component add rustfmt --toolchain=stable), which is invalid; fix it by removing
the run from that step and creating a new standalone step that only runs the
command "rustup component add rustfmt --toolchain=stable" (ensure the new step
appears after the setup-python step and uses only the run key), referencing the
existing actions/setup-python step to keep ordering.

Verify each finding against the current code and only fix it if needed.

Inline comments:
In @.github/workflows/ci.yaml:
- Around line 50-53: The Miri job is using an undefined env var `${{
env.NIGHTLY_CHANNEL }}` and several "stable" jobs install stable but don't force
it for subsequent cargo commands; update the Miri step that runs `cargo +${{
env.NIGHTLY_CHANNEL }} miri test` to use a concrete toolchain (either `cargo
+nightly miri test` or `cargo +${{ env.RUSTUP_TOOLCHAIN }} miri test`) and
ensure the MUST-FIX env is `RUSTUP_TOOLCHAIN: nightly` where that job sets it;
for each job that installs `stable` (where `rustup toolchain install stable` is
run) either add `RUSTUP_TOOLCHAIN: stable` at the job-level env or change
subsequent invocations like `cargo clippy`, `cargo shear`, and other `cargo ...`
steps to explicitly run `cargo +stable ...` so they deterministically use the
stable toolchain.

---

Outside diff comments:
In @.github/workflows/ci.yaml:
- Around line 414-427: The Miri test step uses an undefined env var
NIGHTLY_CHANNEL; update the "Run tests under miri" step to use the defined
RUSTUP_TOOLCHAIN variable (or the literal "nightly") so the cargo +<toolchain>
invocation expands correctly — change the cargo command that references
env.NIGHTLY_CHANNEL to reference env.RUSTUP_TOOLCHAIN (or hardcode "nightly") in
that step.

Verify each finding against the current code and only fix it if needed.

Inline comments:
In @.github/workflows/ci.yaml:
- Line 53: Replace all uses of the reusable action reference
dtolnay/rust-toolchain that use mutable tags (e.g., `@stable` and `@master`) with
their corresponding full 40-character commit SHAs to avoid mutable refs; update
each occurrence (the instances currently written as
dtolnay/rust-toolchain@stable and dtolnay/rust-toolchain@master) to the exact
commit SHA and add a short comment on the same line preserving the original
tag/intent (for example: # originally `@stable`) so CI remains reproducible and
reviewers know the intended channel.

In @.github/workflows/release.yml:
- Line 55: Replace both mutable references of the dtolnay/rust-toolchain action
("uses: dtolnay/rust-toolchain@stable") with the repository's immutable
40-character commit SHA for the current stable tag and add an inline comment
noting which stable tag the SHA corresponds to; locate and update the two
occurrences in the release workflow where "uses: dtolnay/rust-toolchain@stable"
appears, fetch the current commit SHA from the dtolnay/rust-toolchain GitHub
repo for the stable tag, and use that SHA in place of "@stable" so the workflow
is pinned to a specific commit.