Bug 1351031 - Update rust-build container. r=mshal

Update rust-build docker image source to v0.4.3. This includes
some recent to how verification and taskcluster work to maintain
working scripts, and some changes for the newer cargo-building-
cargo build system, which aren't sufficient for bootstrapping.

MozReview-Commit-ID: 4IdbKVvco8m

taskcluster/docker/rust-build/splat_rust.py

@@ -19,8 +19,7 @@ def load_manifest(path):
 
 def save_manifest(manifest, path):
     with open(path, 'w') as f:
-        json.dump(manifest, f,
-                  indent=0, separators=(',', ': '))
+        json.dump(manifest, f, indent=2, separators=(',', ': '))
         f.write('\n')