[release/1.4.15] build: improve debian packaging

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

(cherry picked from commit 4f7627972e)

build/deb.changelog

@@ -1,5 +1,5 @@
 {{.Name}} ({{.VersionString}}) {{.Distro}}; urgency=low
 
-  * git build of {{.Commit}}
+  * git build of {{.Env.Commit}}
 
  -- {{.Author}}  {{.Time}}