spec: fix path to `installing_core` stamp file for subpackages
The other scriptlets which inspect or delete this file expect it to have
the +SUBPKG
suffix. Looking at the original patch introducing this
line, I think that was the intent, but here we need to use %{?-v}
instead of %{?1}
since unlike in the other places the file is
referenced, the subpackage is a flagged macro parameter rather than
positional.
Fixes e8151314 ("redhat: spec: trigger dracut when modules are installed separately").
Fixes: https://github.com/coreos/rpm-ostree/issues/4057
Signed-off-by: Jonathan Lebon jonathan@jlebon.com