ci: give dtolnay/rust-toolchain a valid toolchain input (#219)

## What was wrong

`dtolnay/rust-toolchain@v1` declares `toolchain` as required with no
default (only branches like `@stable` supply one). Affected steps either
set none (`'toolchain' is a required input`), set an invalid value
(`v1`), or carry a second `with:` key injected above the real one by an
earlier bulk edit. A duplicate key makes GitHub reject the whole
workflow file.

## Changes

Each affected step now has exactly one `with:` and a valid toolchain.

- `cargo-audit.yml`, `coverage.yml`, `release.yml`: drop the injected
duplicate `with:`/`toolchain: v1` (the real `toolchain: stable` stays).
- `e2e.yml`: `v1` → `stable` on one step; add `toolchain: stable` to
three others.
- `chapel-ci.yml`: `v1` → `stable` on one step; add it to another.

## Not fixed here

`cargo-audit.yml`, `chapel-ci.yml`, `coverage.yml`, `e2e.yml` and
`release.yml` are also blocked by older `actions.lock` mismatches
(actions bumped without regenerating the lock). GitHub won't start them
until the lock is regenerated; that is separate and unchanged.

## Testing

YAML parses; standards' `check-lock-sync.sh` reports no new failures.

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_Generated by [Claude
Code](https://claude.ai/code/session_0132Cdz3iwk2QGKv2sUhhsye)_

Co-authored-by: Claude <noreply@anthropic.com>

Author: hyperpolymath