### 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>