helpers: fix install race

helpers: fix install race

The install hook needs to be a install-data-hook not an install-exec-hook as the
helpers are installed into helperdir which is considered data (only path
variables with "exec" in are considered executables).

The explicit dependency on install-helpersPROGRAMS was an attempt at solving
this, but this causes occasional races where install-helpersPROGRAMS can run
twice in parallel (once via install-all, once via the hook's dependency).

https://bugzilla.gnome.org/show_bug.cgi?id=758029