fix code coverage windows

[piotrkwiecinski]

On windows path to grcov is:

info: grcov installed at /c/Users/runneradmin/.install-action/bin/grcov.exe
+ grcov --version
grcov 0.8.19

On unix:

info: grcov installed at /home/runner/.cargo/bin/grcov
+ grcov --version
grcov 0.8.19

In both cases they should be available in $PATH

Tested on my fork: https://github.com/piotrkwiecinski/coreutils/actions/runs/8926781245/job/24518622684?pr=7

Fixes #6331

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/timeout/timeout is no longer failing!

[BenWiederhake]

Fantastic! The CI failure is #6275 and #6333, so not a blocker.