snapcraft builds: rewrite build_remote.py to be resilient to snapcraft output changes

[wgreenberg]

Our build_remote.py tooling is highly coupled to snapcraft's stdout format, which is liable to change at any time (and has). We should rewrite it so our builds are more resilient to changes to that output.

For now, we've hardcoded the SNAPCRAFT_REMOTE_BUILD_STRATEGY environment variable to force snapcraft to use the legacy tooling, but this should be considered a temporary fix.