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>

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "@actions/attest",
-  "version": "1.5.0",
+  "version": "1.6.0",
   "lockfileVersion": 2,
   "requires": true,
   "packages": {
     "": {
       "name": "@actions/attest",
-      "version": "1.5.0",
+      "version": "1.6.0",
       "license": "MIT",
       "dependencies": {
         "@actions/core": "^1.11.1",