duneci-install-module: do not use feature branch logic for default branch

If you had a default branch "master" setting DUNECI_BRANCH=xy, then one would get the "master" branch of dependencies installed via duneci-install-module as they most likely have a branch with this name. However we want to get the "xy" branch in this case.

This change disables the logic for the default branch, so one will get "xy" as expected.