Let Nexus see schema_dir to self-update

[smklein]

See:

omicron/smf/nexus/multi-sled/config-partial.toml

Lines 21 to 25 in 33d4f82

	# TODO: Uncomment the following lines to enable automatic schema
	# migration on boot.
	#
	# [schema]
	# schema_dir = "/var/nexus/schema/crdb"

and the context from: #8845 (comment)

If we want Nexus to update itself, we'll need to tweak the configuration to let it see the schema directory.

This should be a small fix, but it'll cause the update-on-boot behavior.

This needs to be coordinated with the other changes for RFD 588.