When actions were SHA-pinned in #341, the tool name was lost. The
original uses: taiki-e/install-action@cargo-llvm-cov passed the tool
name as the tag. After pinning to a SHA, the tool input must be
specified explicitly via with: tool: cargo-llvm-cov.
Co-authored-by: jpoehnelt-bot <jpoehnelt-bot@users.noreply.github.com>