During [1], it was noticed that the tree names differ between variants
for the same Brew build as they come from different trigger
configurations. This is problematic as the tree names are linked to
Checkouts, which are common for all variants for Brew builds.

Allow this to be fixed by specifying the tree name ('name' pipeline
variable) explicitely.

[skip coverage check]

[1] umb-messenger#29



Signed-off-by: Michael Hofmann <mhofmann@redhat.com>