agl-compositor.service.in: Add default EnvironmentFile

agl-compositor.service.in: Add default EnvironmentFile

Debugging compositor protocol requires passing environmental variable
to the compositor. This adds /etc/default/agl-compositor file which was
in the past under /etc/default/weston filename, for this same purpose.

Bug-AGL: SPEC-4219

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Ia6231c6360625f0e194a246d0c392a0a05e8a5d9
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/27084
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>