Verify clippy_utils and rust-toolchain.toml match nightly versions

[BD103]

Maybe write a script, that checks that this version, the one in the README and the clippy_utils version match. Should be possible with some cargo metadata magic and pulling from crates.io. Something like this:

VERSION=$(cargo metadata --format-version 1 | jq '.packages[] | select(.name == "clippy_utils")' | jq -r '.version')
EXP_NIGHTLY=$(wget -qO- https://crates.io/api/v1/crates/clippy_utils/${VERSION}/readme | grep -Po "nightly-\d\d\d\d-\d\d-\d\d")

RTC_NIGHTLY=$(grep -Po "nightly-\d\d\d\d-\d\d-\d\d" rust-toolchain.toml)
RM_NIGHTLY=$(grep -Po "nightly-\d\d\d\d-\d\d-\d\d" bevy_lint/README.md | head -1)

test $EXP_NIGHTLY = $RTC_NIGHTLY
test $EXP_NIGHTLY = $RM_NIGHTLY

The RM_NIGHTLY command needs to be adjusted, as there are other nightly mentions in the file. The rest should work with maybe some minor adjustments.

Originally posted by @flip1995 in #186 (comment)