ci: use amber-phone-staging repo for x86 build

4 jobs for bump_valac in 22 minutes and 12 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Package
passed #313883
librem5:arm64
package:deb-debian-buster:arm64

00:21:08

passed #313882
librem5
package:deb-pureos-amber

00:02:40

 
  Test Package
passed #313884
librem5
autopkgtest-debian-buster-package

00:00:39

passed #313885
librem5
lintian-debian-buster-package

00:00:44