[Feature]: elpaca-process-poll unnecessarily creates a emacs subprocess

[Alan-Chen99]

Feature Description

elpaca-process-poll currently runs a emacs subprocess which runs the target process.
A subprocess is already sufficiently isolated so this is not needed.

Confirmation

• The feature I'm proposing does not already exist in Elpaca

[progfolio]

You're correct. The only place this is currently used is in elpaca--run-build-commands.
Eventually I'd like to completely remove the :pre-build and :post-build recipe keywords, replacing them with tools which make it easier to manipulate the :build keyword. That should be conceptually simpler.