More robustly unspoof HOME in Eglot tests (bug#61637)

More robustly unspoof HOME in Eglot tests (bug#61637)

A fair number of LSP servers allow user-local installations instead of
system wide installations.  Emacs's technique of spoofing the HOME env
var to some non-existent or non-home directory during 'make check'
breaks these tests.  That's because the executables are still found by
executable-find, but their invocation will rarely be successful as
HOME isn't what they expect it to be.

Eglot tests already had a technique for dealing with this,
"unspoffing" HOME just for the invocations of LSP server but it
stopped working a while back.  So make it more robust.

Eventually, we'll want to decide wether these local servers should be
considered in 'make check' runs at all, or whether there is a way to
use them with a spoofed HOME.

* test/lisp/progmodes/eglot-tests.el (eglot--call-with-fixture):
More robustly unspoof HOME.