ci: stabilize the Rust cache key by removing the runner's bundled toolchain

[lwshang]

Problem

Swatinem/rust-cache folds the version of every installed Rust toolchain into its environment hash, which is part of the cache key. GitHub runner images ship a stable toolchain whose version drifts as the image is updated — so the cache key silently changes from run to run, jobs miss the cache, and rebuild from cold.

This was observed directly while debugging the Windows test cache: two otherwise-identical builds produced different env hashes (…-9fc860dc-… vs …-b9079240-…) with the same Cargo.lock, i.e. the only thing that moved was the bundled toolchain version.

Fix

We already pin the toolchain via rust-toolchain.toml (1.95.0), so the runner's bundled stable is never actually used for builds — it only pollutes the cache key. This PR removes it before setup-rust-toolchain in every workflow that caches:

- name: Remove the runner's bundled Rust toolchain
  run: rustup toolchain remove stable 2>/dev/null || true

Added before all 8 setup-rust-toolchain usages:

• test.yml (2 — unit-tests + test)
• checks.yml (5 — compile, lint, format, cli-ref, icp-yaml-schema)
• validate-examples.yml (1)

The step is non-fatal (2>/dev/null || true) and runs under the default bash shell on every runner it touches.

Notes for reviewers

• Expect one more cold prime per cache namespace (-test, -checks, -validate) when this merges — removing stable shifts the env hash once more, to whatever the 1.95.0-only environment produces. After that the key should stay put run-to-run, which is the whole point.
• Follow-up to ci: speed up Windows integration tests #595 (Windows test speedups). Same rollout logic applies: once it's on main, every PR (base = main) reuses the new, stable-keyed caches.

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