elements/components/dummy-gbm.bst: append to install-commands instead of replacing

We would have an empty artifact otherwise
10 jobs for abderrahim/xorg-gbm-fix in 166 minutes and 19 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Flatpak
passed #251004225
aarch64
app_aarch64

02:14:28

passed #251004226
armhf
app_arm

01:19:27

passed #251004224
cache_i686 x86_64
app_i686

01:48:30

passed #251004227
ppc64le allowed to fail
app_powerpc64le

00:56:32

passed #251004223
cache_x86_64 x86_64
app_x86_64

01:34:22

 
  Vm
manual #251004230
aarch64 allowed to fail manual
minimal_systemd_vm_aarch64
passed #251004231
armhf
minimal_systemd_vm_arm

00:27:57

passed #251004229
cache_i686 x86_64
minimal_systemd_vm_i686

00:31:47

manual #251004232
ppc64le allowed to fail manual
minimal_systemd_vm_powerpc64le
passed #251004228
cache_x86_64 x86_64
minimal_systemd_vm_x86_64

00:31:44