Upgrade bazel to 2.1.0 - Githubissues

Upgrade us to a newer version of bazel. Fix incompatibilities along the way.

Also updates the debian-iso version and adds a helper script for that, because that got broken along the way.

Fixes #444.

Checklist:

[x] I have split up this change into one or more appropriately-delineated commits.
[x] The first line of each commit is of the form "[component]: do something"
[x] I have written a complete, multi-line commit message for each commit.
[x] I have formatted any Go code that I have changed with gofmt.
[x] I have written or updated appropriate documentation to cover this change.
[x] I have confirmed that this change is covered by at least one appropriate test run by CI.
[x] If my change includes new or modified functionality, I have tested that the changes work as expected.
[x] I have assigned this issue to an appropriate reviewer. (Choose @celskeggs if you are not otherwise certain.)
[x] I consider my PR complete and ready to be merged without my further input, assuming that it passes CI and code review.
[x] My changes have passed CI, including an automatic Jenkins deploy.
[x] My changes have passed code review.

sipb / homeworld