code coverage is broken - details of the error

[sylvestre]

I am going to collect some finding of my work (@cakebaker probably has other).

• -Zprofile has been removed from rustc - Remove support for -Zprofile (gcov-style coverage instrumentation) rust-lang/rust#131829
  we should use -Cinstrument-coverage instead

• we have to switch to a different method described here readme: example uses -Zprofile which has been removed from Rust nightly mozilla/grcov#1240 (cc @marco-c)

• we need to pass LLVM_PROFILE_FILE as a new env variable, which wasn't passed to some commands by ucmd()
  i fixed it in 0cd58c6
  it was leaving some coverage artifacts in the pwd which was breaking some tests like ls.
  LLVM_PROFILE_FILE: "${{ github.workspace }}/build/coverage-%p-%m.profraw"

• because of the change of format, files have to be exported into a different directory to make sure they don't pollute the tests

• the coverage files are HUGE
  56G /Users/runner/work/coreutils/coreutils/build/ on a mac
  it causes the linux machine to fails with
  System.IO.IOException: No space left on device : '/home/runner/runners/2.320.0/_diag/Worker_20241116-105056-utc.log'
  and doesn't show anything in the log

• it takes 16 minutes for grcov to process everything - it was a few seconds before

• During one of my numerous experiment, i got <LLVM Profile Warning: Unable to track new values: Running out of static counters. Consider using option -mllvm -vp-counters-per-site=<n> to allocate more value profile counters at compile time. despite passing -Clink-arg=-Wl,-mllvm -Clink-arg=-Wl,-vp-counters-per-site=6, still happened.

• Using cargo llvm-cov show-env like in https://github.com/foundationdb-rs/foundationdb-rs/blob/c6623a84a7f35597eb7f68357a3c9cb91831501e/.github/workflows/coverage.yml fails the same way

My experimentations: #6850