[remote-build] source-type 'zip' aborts with KeyError

[glancr team]

I have a snapcraft.yaml where multiple parts have their source as ZIP files on remote URLs. As these do not have a .zip suffix, snapcraft requires `source-type: zip` as a hint.

The snap builds fine locally or on launchpad builders. However, doing a remote-build fails with:

Using snapcraft 4.3 both on macOS 10.15.7 and Ubuntu 20.04 LTS

```
Sorry, an error occurred in Snapcraft:
'zip'
Traceback (most recent call last):
  File "/usr/local/bin/snapcraft", line 8, in <module>
    sys.exit(run())
  File "/usr/local/Cellar/snapcraft/4.3/libexec/lib/python3.8/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/Cellar/snapcraft/4.3/libexec/lib/python3.8/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/usr/local/Cellar/snapcraft/4.3/libexec/lib/python3.8/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/Cellar/snapcraft/4.3/libexec/lib/python3.8/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/Cellar/snapcraft/4.3/libexec/lib/python3.8/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/usr/local/Cellar/snapcraft/4.3/libexec/lib/python3.8/site-packages/snapcraft/cli/remote.py", line 163, in remote_build
    _start_build(
  File "/usr/local/Cellar/snapcraft/4.3/libexec/lib/python3.8/site-packages/snapcraft/cli/remote.py", line 194, in _start_build
    repo_dir = wt.prepare_repository()
  File "/usr/local/Cellar/snapcraft/4.3/libexec/lib/python3.8/site-packages/snapcraft/internal/remote_build/_worktree.py", line 326, in prepare_repository
    part_config = self._process_part_sources(part_name, part_config)
  File "/usr/local/Cellar/snapcraft/4.3/libexec/lib/python3.8/site-packages/snapcraft/internal/remote_build/_worktree.py", line 245, in _process_part_sources
    part_config["source"] = self._pull_source(part_name, source)
  File "/usr/local/Cellar/snapcraft/4.3/libexec/lib/python3.8/site-packages/snapcraft/internal/remote_build/_worktree.py", line 163, in _pull_source
    source_handler = self._get_part_source_handler(part_name, source, download_dir)
  File "/usr/local/Cellar/snapcraft/4.3/libexec/lib/python3.8/site-packages/snapcraft/internal/remote_build/_worktree.py", line 92, in _get_part_source_handler
    handler_class = snapcraft.internal.sources.get_source_handler(
  File "/usr/local/Cellar/snapcraft/4.3/libexec/lib/python3.8/site-packages/snapcraft/internal/sources/__init__.py", line 170, in get_source_handler
    return _source_handler[source_type]
KeyError: 'zip'
```

Building the same snapcraft.yaml through remote-build worked pre-snapcraft 4.3.