core/exec-invoke: suppress placeholder home only in build_environment()

core/exec-invoke: suppress placeholder home only in build_environment()

Currently, get_fixed_user() employs USER_CREDS_SUPPRESS_PLACEHOLDER,
meaning home path is set to NULL if it's empty or root. However,
the path is also used for applying WorkingDirectory=~, and we'd
spuriously use the invoking user's home as fallback even if
User= is changed in that case.

Let's instead delegate such suppression to build_environment(),
so that home is proper initialized for usage at other steps.
shell doesn't actually suffer from such problem, but it's changed
too for consistency.

Alternative to #34789

(cherry picked from commit b718b86e1b8477f58461f3c456c944abb1428c0f)