zig.eclass: fix rare spurious `error.BrokenPipe` during `src_test`

Most likely caused by `grep -q` processing too fast when steps are
filtered. Can sometimes be reproduced with this command (you might
need several retries to hit it):

```console
$ for i in {0..20}; do zig build --list-steps | grep -q test; done
error: BrokenPipe
/usr/lib64/zig/9999/lib/compiler/build_runner.zig:1229:9: 0x1518f3e in steps__anon_4736 (build)
        try out_stream.print("  {s:<28} {s}\n", .{ name, top_level_step.description });
        ^
/usr/lib64/zig/9999/lib/compiler/build_runner.zig:374:9: 0x1511b81 in main (build)
        return steps(builder, stdout_writer);
        ^
error: the following build command failed with exit code 1:
/home/bratishkaerik/github.com/zig/.zig-cache/o/4b3846557c333ec9e467b7cc136d3698/build /usr/lib64/zig/9999/bin/zig /usr/lib64/zig/9999/lib /home/bratishkaerik/github.com/zig /home/bratishkaerik/github.com/zig/.zig-cache /home/bratishkaerik/.cache/zig --seed 0x86f5c718 -Z7b5e4ad814524daf --list-steps
```

Caught this today when I merged and tested `dev-lang/zig:9999` 9 times
in a row because I was checking patch for upstream. Out of these 9 times
it failed only once. I couldn't reproduce this on `sys-fs/ncdu` or
`gui-wm/river::guru`, my guess here is that `test` step is listed last
there, so zig pipe catches up with it.

Signed-off-by: Eric Joldasov <bratishkaerik@landless-city.net>
Closes: https://github.com/gentoo/gentoo/pull/39723
Signed-off-by: Sam James <sam@gentoo.org>

0 files changed, 0 insertions, 0 deletions