FindPkgConfig: Fix preservation of ENV{PKG_CONFIG_ALLOW_SYSTEM_LIBS}
Fix the ENV save/restore added by !6345 (merged) to unset the variable if it was originally not set.
While at it, clarify the value to which we temporarily set it. The
pkg-config
and pkgconf
tools only check that the variable is set,
and do not care about the value. Set it to a more true-looking value.
Issue: #22148 (closed)
Backport: release