Fix buildcache docs for read-only cache

[RMeli]

The documentation states that

A build cache can be configured to be read-only by not providing a key in the cache configuration file.

However, not providing a key results in:

'key' is a required property

Failed validating 'required' in schema:
    {'$schema': 'http://json-schema.org/draft-07/schema#',
     'additionalProperties': False,
     'properties': {'key': {'default': None,
                            'oneOf': [{'type': 'string'},
                                      {'type': 'null'}]},
                    'root': {'type': 'string'}},
     'required': ['key', 'root'],
     'title': 'Schema for Spack Stack cache.yaml',
     'type': 'object'}

This PR removes key from the required keywords, since it has a default value null.

[RMeli]

Actually, this fails later on with

==> Selected 23 specs to push to file:///bret/scratch/cscs/rmeli/spack-cache/user-environment
==> Error: No default key available for signing.