Update Rust crate cargo-llvm-cov to v0.6.16

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
cargo-llvm-cov	dev-dependencies	patch	0.6.15 -> 0.6.16

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Release Notes

taiki-e/cargo-llvm-cov (cargo-llvm-cov)

v0.6.16

Compare Source

• Add --with-pwsh-env-prefix to cargo llvm-cov show-env subcommand. (#​411, thanks @​LittleBoxOfSunshine)

• Document usage with GitLab CI. (#​405, thanks @​jaskij)

• Document usage with afl.rs. (#​369, thanks @​njelich)

• Update ruzstd to 0.8.

  This increases the rustc version required to build cargo-llvm-cov. (rustc 1.73+ -> 1.81+)
  The cargo/rustc version required to run cargo-llvm-cov remains unchanged.

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