NEED REVIEW : Correct workflow yml files

[MykolaMS9]

Task

Update GitHub Actions workflows to:

• Add the ability to choose a custom (e.g. self-hosted) runner label instead of the hardcoded ubuntu-latest.
• Pin all third-party actions to a specific commit SHA instead of a mutable version tag, to protect against supply-chain attacks (a tag can be moved to point at different, possibly malicious, code).

Solution

• Replaced runs-on: ubuntu-latest with runs-on: ${{ vars.GH_RUNNER_LABEL_CI || 'ubuntu-latest' }} in ci.yml, students_ci.yml and students_tool_cache.yml. This lets CI switch to a custom runner by setting the GH_RUNNER_LABEL_CI repository/organization variable, while still defaulting to ubuntu-latest when it's not set (GitHub Actions expressions treat an unset vars.* as an empty, falsy string, so || works as a default-value operator here).
• Pinned every uses: action reference to its full commit SHA, with a # <version> comment for readability:
  • actions/checkout@v3/@v4 → @9c091bb... (v7.0.0)
  • gaurav-nelson/github-action-markdown-link-check@v1 → @3c3b66f... (1.0.17) — the v1 tag turned out to be frozen on a 2022 commit and no longer tracks the maintainer's actual releases.
  • Swatinem/rust-cache@v2 → @e18b497... (v2.9.1)
• Replaced actions-rs/toolchain@v1 with dtolnay/rust-toolchain (pinned to SHA), because the actions-rs GitHub org was archived in October 2023 and the action is unmaintained (relies on deprecated Node.js runtimes). Updated the dependent cache key from steps.rust.outputs.rustc_hash to steps.rust.outputs.cachekey, since the new action exposes the cache key under a different output name.