Restore coverage to use latest nightly (#3450)

Fixes #3367

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

.github/workflows/coverage.yml

@@ -31,10 +31,7 @@ jobs:
 
       - uses: actions-rs/toolchain@v1.0.7
         with:
-          # Pinned to workaround issue making cargo-llvm-cov fail, see
-          # https://github.com/taiki-e/cargo-llvm-cov/issues/128
-          # TODO: restore to just `nightly` after it's fixed
-          toolchain: nightly-2022-01-14
+          toolchain: nightly
           override: true
           profile: minimal
           components: llvm-tools-preview