I am more inclined to have a way to pass credentials or VCS (including ssh) into the environments.
This could be as simple as mounting .ssh .git and such into the build environments or parametrize somehow through the snapcraft cli.
While I haven’t tried yet, when using LXD, you could coherse the default profile to add those disk devices by default, snapcraft will not care (we will be moving to a snapcraft specific lxd profile, and maybe use the project feature, in the future to make this a bit more manageable).