chore(ci): override RUSTUP_TOOLCHAIN for the check step (#2116)

Summary
Fixes an issue where the "check" CI jobs for MSRV and stable were
unintentionally using the stable toolchain from `rust-toolchain.toml`
instead of the matrix-specified toolchain.

Details
- Added `RUSTUP_TOOLCHAIN: ${{ matrix.toolchain }}` to the "check" job
in CI configuration.

Additional Context
- https://github.com/ratatui/ratatui/pull/2106
-
https://discord.com/channels/1070692720437383208/1072879985762762812/1422345357131780177
- https://rust-lang.github.io/rustup/overrides.html#overrides

Signed-off-by: Shingo OKAWA <shingo.okawa.g.h.c@gmail.com>

.github/workflows/ci.yml

@@ -165,6 +165,8 @@ jobs:
           tool: cargo-hack
       - uses: Swatinem/rust-cache@f13886b937689c021905a6b90929199931d60db1 # v2
       - run: cargo xtask check --all-features
+        env:
+          RUSTUP_TOOLCHAIN: ${{ matrix.toolchain }}
 
   build-no-std:
     name: Build No-Std