check-invariants: Ignore linux pkg upstream branch merged invariant

Task https://phabricator.apertis.org/T8551

check-invariants: Ignore linux pkg upstream branch merged invariant

Currently, all the downstream channels that are based on the source
branch are checked to be merged. This doesn't match the linux package
configuration, as it doesn't pull updates from the default release
upstream sources.

For instance, apertis/v2021 release is based on debian/buster, but the
apertis/v2021 linux package pulls from debian/bullseye.

Let's ignore the linux package upstream branches checking invariant as
this is reporting wrong entries and there's another script which takes
care of handling all linux package updates:

  ./bin/packaging-updates-upstream-linux