set workflow.ref provenance field from ref claim

Updates the `buildSLSAProvenancePredicate` function to populate the
`workflow.ref` field from the `ref` claim in the OIDC token.

Signed-off-by: Brian DeHamer <bdehamer@github.com>

packages/attest/__tests__/provenance.test.ts

@@ -75,16 +75,6 @@ describe('provenance functions', () => {
       const predicate = await buildSLSAProvenancePredicate()
       expect(predicate).toMatchSnapshot()
     })
-
-    it('handle tags including "@" character', async () => {
-      nock.cleanAll()
-      await mockIssuer({
-        ...claims,
-        workflow_ref: 'owner/repo/.github/workflows/main.yml@foo@1.0.0'
-      })
-      const predicate = await buildSLSAProvenancePredicate()
-      expect(predicate).toMatchSnapshot()
-    })
   })
 
   describe('attestProvenance', () => {