chore(ci): override the toolchain for CI runs (#2106)

### Summary
Fixes an issue where the beta CI jobs were unintentionally using the
stable
toolchain from `rust-toolchain.toml` instead of the matrix-specified
toolchain.
This caused clippy to run against the wrong version.

### Details
- Added `RUSTUP_TOOLCHAIN: ${{ matrix.toolchain }}` to the CI
configuration

### Additional Context
-
https://discord.com/channels/1070692720437383208/1072879985762762812/1421990770482745415
- 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

@@ -104,6 +104,8 @@ jobs:
           components: clippy
       - uses: Swatinem/rust-cache@98c8021b550208e191a6a3145459bfc9fb29c4c0 # v2
       - run: cargo xtask clippy
+        env:
+          RUSTUP_TOOLCHAIN: ${{ matrix.toolchain }}
 
   # Run markdownlint on all markdown files in the repository.
   lint-markdown: