Optimize child process spawns by skipping job server configs

[rnk]

Configuring the job server so that all child processes can access it causes Rust's subprocess library to switch from posix_spawn to fork, which supports running arbitrary code in a pre-exec context. Fork is, unfortunately, very expensive. You really want to use vfork or posix_spawn, which IIRC uses that under the hood, in order to get fast process launching.

This really doesn't matter in the grand scheme of things because sccache direct mode (the defatult) doesn't launch a ton of processes when it gets a cache hit, but if you turn it off, it will launch may pre-processing jobs, and the overhead is observable in a profiler, which is how I (well, Claude) found this:

Measured on a 2456-file LLVM rebuild (X86 only, Release, clang, -j16,
16 cores) where every compile is a cache hit, so the preprocessor spawn is
essentially all the server does. Four interleaved rounds per arm:

| arm    | wall             | server CPU       | of which system |
|--------|------------------|------------------|-----------------|
| before | 40.7 s (+-0.7)   | 28.2 s (+-0.4)   | 22.8 s          |
| after  | 33.5 s (+-0.2)   | 10.6 s (+-0.1)   |  5.8 s          |

It seemed like a reasonably small contribution that might make a good first PR.

The code is AI generated, and I'll take another pass to try to simplify it. I think it has the wrong job server default.

[sylvestre]

Nice to see you here :)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.88889% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.73%. Comparing base (8ab3926) to head (3e3d27e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/mock_command.rs	85.71%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2827      +/-   ##
==========================================
+ Coverage   73.71%   73.73%   +0.01%     
==========================================
  Files          72       72              
  Lines       37932    37976      +44     
==========================================
+ Hits        27963    28002      +39     
- Misses       9969     9974       +5     

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