FIx Multipass snap lint issues and lint snap in CI

[ricab]

Here's snapcraft's lint output for a recent multipass snap:

$ snapcraft lint multipass_17610.snap
Running linter.
Running linter.
Lint warnings:
- library: libdart_ffi.so: unused library 'usr/lib/libdart_ffi.so'. (https://documentation.ubuntu.com/snapcraft/stable/how-to/debugging/use-the-library-linter)
- library: libvirt.so.0: unused library 'usr/lib/x86_64-linux-gnu/libvirt.so.0.8000.0'. (https://documentation.ubuntu.com/snapcraft/stable/how-to/debugging/use-the-library-linter)
- metadata: Metadata field 'contact' is missing or empty. (https://documentation.ubuntu.com/snapcraft/stable/reference/project-file/snapcraft-yaml/#contact)
Lint information:
- metadata: Metadata field 'issues' is missing or empty. (https://documentation.ubuntu.com/snapcraft/stable/reference/project-file/snapcraft-yaml/#issues)
- metadata: Metadata field 'source-code' is missing or empty. (https://documentation.ubuntu.com/snapcraft/stable/reference/project-file/snapcraft-yaml/#source-code)
- metadata: Metadata field 'website' is missing or empty. (https://documentation.ubuntu.com/snapcraft/stable/reference/project-file/snapcraft-yaml/#website)