Figure out why pipes in compiletest breaks

[eholk]

I had to revert the change to use pipes in compiletest (96c6f57). We were having build failures with an invalid argument error in lock_and_signal. This usually means we are trying to acquire an already-freed lock. We need to figure out why this happens.

[eholk]

This was due to a race condition which has been fixed by reference counting the task during select.

[rust-log-analyzer]

The job x86_64-gnu-llvm-15 failed! Check out the build log: (web) (plain)

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##[endgroup]
Skipping Set({test::src/tools/tidy}) because it is excluded
##[group]Building stage0 library artifacts (x86_64-unknown-linux-gnu)
    Updating crates.io index
error: the lock file /checkout/Cargo.lock needs to be updated but --locked was passed to prevent this
If you want to try to generate the lock file without accessing the network, remove the --locked flag and use --offline instead.
  local time: Mon Aug 21 10:07:01 UTC 2023
  network time: Mon, 21 Aug 2023 10:07:01 GMT
##[error]Process completed with exit code 1.
Post job cleanup.