Use cargo xtask fmt --check in CI

[nicholasbishop]

In #1196, it was noticed that some files aren't formatted in the way cargo xtask fmt would have done it. This happened even though we check formatting in CI, and the reason is that in CI we are running cargo fmt --all -- --check instead of cargo xtask fmt --check, which adds additional args.

uefi-rs/.github/workflows/rust.yml

Line 91 in a70ccab

cargo fmt --all -- --check