Don't install Build-Depends-Indep packages in the test-build-any job

Hi! Thanks for providing this service, it is really wonderful.

I recently git build failures on buildd's that I didn't in salsa-ci, compare:

• https://buildd.debian.org/status/fetch.php?pkg=gss&arch=arm64&ver=1.0.3-5&stamp=1610141484&raw=0
• https://salsa.debian.org/auth-team/gss/-/pipelines/217017

If I understand correctly, the reason is that buildd's on each architecture only does binary-arch builds and does not include binary-indep stuff.

It seems logical to add new 'build indep' target in the 'test' stage that does a binary-indep-only build.

Before starting to become familiar with the implementation and provide a patch, I wanted to file this bug to see if anyone with more knowledge agrees with me, or if I'm missing something.

Thoughts?

Thanks, Simon