Toolchain no longer needs to be pinned for coverage

[iamwacko]

Is your feature request related to a problem? Please describe.
Currently running coverage.sh will install nightly-2022-01-09. This is unnecessary, as rust-lang/rust#79645 and taiki-e/cargo-llvm-cov#128 are closed.

Describe the solution you'd like
Remove the parts of coverage.sh relating to adding

Describe alternatives you've considered
There aren't really any alternatives

Additional context
This doesn't really directly affect me, but I thought you guys should know.

[cwaldren-ld]

Hey @iamwacko , we appreciate the notice. This has been resolved internally and will be fixed in the next release.