This removes the $PIP_INSTALL environment variable in favor of a new sdh_pip_install helper function in sage-dist-helpers. This is consistent with the purpose of the library of build helper functions, and is hopefully the last time such a mass change should be made.

(I realize in #21441 comment:36 I argued against making more mass updates to replace PIP_INSTALL, but I'm not wild about the environment variable either, and now that the helper functions exist this seems like the best approach for consistency's sake.)

The only change this makes to the spkg-install scripts is a bulk sed.

Component: build

Author: Erik Bray

Branch/Commit: c69dfda

Reviewer: Jeroen Demeyer

Issue created by migration from https://trac.sagemath.org/ticket/24014