Add check for `toolchain`

[MrAlias]

Similar to open-telemetry/opentelemetry-go#5983, add a CI check to ensure the toolchain directive is not added to any go.mod in the repository.

Originally posted by @pellared in open-telemetry/opentelemetry-go#5983 (comment)

[7h3-3mp7y-m4n]

I'll take it, Thanks!