tests: run gccgo only on ubuntu-16.04-64

[fgimenez]

We only need to check that snapd builds with gccgo in one architecture.

[zyga]

It would be good to add rationale why this is happening.

[fgimenez]

@zyga done, thanks!

[mvo5]

@fgimenez did you maybe forgot a push here? I see no rational in the diff. The change itself looks fine, +1

[fgimenez]

@mvo5 yep sorry, i only added it to the PR description, pushed as a comment too, thanks!