Switch to Unix.create_process for auxiliaries

[dra27]

Historically, flexlink eschews using the Unix library, but this is daft - we're going through ridiculous contortions to work around command line length limits, and falling back to bash has become ever more brittle.

Motivated by the issue reported on Discuss

[Alizter]

This (the fallback to bash) appears to happen in dune's bootstrap. See the issue here https://github.com/ocaml/dune/issues/11574#issuecomment-2835894536

I'm not sure why bash is considered a fallback for command length limits however. Surely that is one thing you can't expect to find on windows?

[nojb]

I'm not sure why bash is considered a fallback for command length limits however. Surely that is one thing you can't expect to find on windows?

In practice, until recently, I dare to say that most Windows users of OCaml did so via Cygwin.

[nojb]

Historically, flexlink eschews using the Unix library, but this is daft

@dra27 You are the expert, but won't this cause issues when bootstrapping flexlink?

[dra27]

I was expecting to do something akin to ocamlrun and ocamlruns - so there would be an cut-back flexlink in boot/ and then the actual one built to install would be linked with Unix (given that there's already flexlink.opt being built separately, I'm hoping that won't look too bad in the build system)