fix: package.json & package-lock.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-ACTIONSCORE-1015402

package.json

@@ -18,7 +18,7 @@
   },
   "homepage": "https://github.com/nick-invision/retry#readme",
   "dependencies": {
-    "@actions/core": "^1.2.4",
+    "@actions/core": "^1.2.6",
     "milliseconds": "^1.0.3",
     "tree-kill": "^1.2.2"
   },