I didn't add this initially because the command I tried was too slow.
The 'git for-each-ref ...' invocation takes 40ms on my machine. That
ought to be acceptable.
This commit tweaks the debian packaging tool:
* All build environment metadata can now be overriden on the command
line. This allows testing the CI build behaviour locally.
* -unstable packages now actually contain the binaries (oops)
* packages use Go 1.7 to build
* archiving is skipped for PR builds
