add actual gha check

[kltm]

for #27399

[kltm]

@pgaudet I've been turning on the schema check on this branch and I've run into some issues that need clarification.

• In go_subsets_metadata.yaml, goslim_virus has two roles; the schema requires a single role
• In go_subsets_metadata.yaml, goslim_goa needs the additional metadata (like contact, role) to conform with the schema, even though it is obsolete; this can be cribbed from the other obsolete subsets?

[kltm]

Noting that this can be merged once the tests pass.

[kltm]

Talking to @pgaudet , we are not using this internally, so it should be safe to make a schema change to allow a multi-valued role

[kltm]

Merging, to prevent drift and the fail is issue #28454