ci: Fix coverage workflow

Due to a regression (?) in the rustc compiler, the code coverage
workflow has been failing since 2025-05-20. Temporarily use a version of
nightly that is known to work, until rust-lang/rust#141577 (or whatever
replaces it) is resolved.

Author: jan-ferdinand

.github/workflows/coverage.yml

@@ -17,7 +17,7 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Install toolchain
-        uses: dtolnay/rust-toolchain@nightly
+        uses: dtolnay/rust-toolchain@nightly-2025-05-18 # date is temporary – see issue #600
         with:
           components: llvm-tools-preview
 
@@ -32,7 +32,7 @@ jobs:
       # at all, documentation tests are skipped.
       - name: Collect coverage data (skip doctests & benchmarks)
         run: >
-          cargo +nightly llvm-cov nextest
+          cargo llvm-cov nextest
           --lib --bins --tests --examples
           --lcov --output-path lcov.info
           --