choose proper sigstore instance when attesting (#11)

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

package.json

@@ -71,6 +71,7 @@
     "@sigstore/oci": "^0.1.0"
   },
   "devDependencies": {
+    "@sigstore/mock": "^0.6.5",
     "@types/jest": "^29.5.12",
     "@types/make-fetch-happen": "^10.0.4",
     "@types/node": "^20.11.21",
@@ -83,6 +84,7 @@
     "eslint-plugin-prettier": "^5.1.3",
     "jest": "^29.7.0",
     "js-yaml": "^4.1.0",
+    "nock": "^13.5.4",
     "prettier": "^3.2.5",
     "prettier-eslint": "^16.3.0",
     "ts-jest": "^29.1.2",