Revert "ci: coverage report running on stable rust"

This reverts commit be45210.

Author: YeungOnion

.github/workflows/coverage.yml

@@ -14,7 +14,7 @@ jobs:
       CARGO_TERM_COLOR: always
     steps:
       - uses: actions/checkout@v4
-      - uses: dtolnay/rust-toolchain@stable
+      - uses: dtolnay/rust-toolchain@nightly
         with:
           components: llvm-tools-preview