ci(lint): pin lint job to Rust 1.88.0 to fix rustfmt-not-installed error

The lint job used `dtolnay/rust-toolchain@stable` which installs
rustfmt + clippy on the current stable (1.95).  But `rust-toolchain.toml`
pins cargo to 1.88.0, so when `cargo fmt --check` runs, it routes
through the 1.88.0 toolchain that lacks rustfmt:

  error: 'cargo-fmt' is not installed for the toolchain
  '1.88.0-x86_64-unknown-linux-gnu'

Switch the lint job to `dtolnay/rust-toolchain@1.88.0` so the
installed components match the toolchain that actually runs cargo.
The build/test jobs still use `@stable` since they only need rustc +
cargo, both of which come with the default 1.88.0 install rustup
auto-fetches when it sees `rust-toolchain.toml`.

Author: UnbreakableMJ

.github/workflows/ci.yml

@@ -48,7 +48,12 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - run: sudo apt-get update && sudo apt-get install -y perl
-      - uses: dtolnay/rust-toolchain@stable
+      # Install on the EXACT toolchain rust-toolchain.toml pins (1.88.0).
+      # `@stable` would install rustfmt/clippy on the latest stable instead,
+      # but `cargo` ends up calling the pinned 1.88.0 toolchain that lacks
+      # those components, producing `'cargo-fmt' is not installed for the
+      # toolchain '1.88.0-x86_64-unknown-linux-gnu'`.
+      - uses: dtolnay/rust-toolchain@1.88.0
         with:
           components: rustfmt, clippy
       - uses: Swatinem/rust-cache@v2