Bug 1399679 - Use debian7-*-build instead of desktop-build. r=gps

Switch almost all builds currently using the desktop-build image to use
the right debian7-*-build image instead. The only exception is the rust
bindgen spidermonkey builds, that require cmake being installed, but I
don't want to add it to the base images because that involves risking a
cmake dependency unwantedly slipping in Firefox (rust-bindgen ironically
requires cmake to build a single C++ file...)

taskcluster/taskgraph/transforms/repackage.py

@@ -161,7 +161,7 @@ def make_job_description(config, jobs):
                 )
 
             run['tooltool-downloads'] = 'internal'
-            worker['docker-image'] = {"in-tree": "desktop-build"}
+            worker['docker-image'] = {"in-tree": "debian7-amd64-build"}
 
         description = (
             "Repackaging for locale '{locale}' for build '"