google backend fails if username contains `@` or `.`

Spread fails with an unfriendly error if the username contains non-standard characters.

This occurs on my corporate-provisioned laptop where my username is an email address:

```bash
> spread google:
2025-04-11 11:35:45 Cannot allocate google:ubuntu-24.04-64: cannot allocate new Google server for ubuntu-24.04-64: invalid value for field 'resource.labels': ''. Label value 'callahan.kovacs@canonical.com' violates format constraints. The value can only contain lowercase letters, numeric characters, underscores and dashes. The value can be at most 63 characterslong. International characters are allowed
```

One workaround is to change `$USER` when running spread:
```bash
USER=hotdog spread google:
```

The issue does not name a file or test; begin by tracing the Google backend's handling of the username and the resource label shown in the error. Confirm the intended behavior for usernames containing @ or . and verify that Google allocation works without requiring a USER override.

Written by the indexing model from the issue text.