Update cargo-llvm-cov to new version with relative paths.

Workaround for failing Win coverage with too-long command.

Rebased on main.

.github/workflows/ci.yml

@@ -265,7 +265,7 @@ jobs:
           host=$(rustc -Vv | grep host | sed 's/host: //')
           curl -fsSL https://github.com/taiki-e/cargo-llvm-cov/releases/download/v${CARGO_LLVM_COV_VERSION}/cargo-llvm-cov-"$host".tar.gz | tar xzf - -C ~/.cargo/bin
         env:
-          CARGO_LLVM_COV_VERSION: 0.1.15
+          CARGO_LLVM_COV_VERSION: 0.1.16
       - run: pip install -U pip nox
       - run: |
           cargo xtask coverage --output-lcov coverage.lcov