Pin nightly version in CI as temporary fix to coverage issue

[YJDoc2]

This PR pins the version of nightly in the CI to fix the error in the CI coverage job, which is occurring. I had opened taiki-e/cargo-llvm-cov#128 where it linked to an closed issue in upstream rust repo, where it is explained that due to some changes in nightly there is some error in generating llvm coverage information.

The current fix is to pin the nightly version to a working version in CI until the nightly version is compatible again.

[codecov-commenter]

Codecov Report

Merging #619 (bfe59e4) into main (8db02e5) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #619      +/-   ##
==========================================
- Coverage   70.09%   70.07%   -0.02%     
==========================================
  Files          82       82              
  Lines       10924    10924              
==========================================
- Hits         7657     7655       -2     
- Misses       3267     3269       +2     

[YJDoc2]

@utam0k PTAL
If this is ok, and merged, other PRs would need to pull and merge the main, which should trigger a re-run, and fix the coverage ; the dependabot PRs should auto-pull and fix themselves.

[utam0k]

Thanks!