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-lock.json

@@ -5,9 +5,9 @@
   "requires": true,
   "dependencies": {
     "@actions/core": {
-      "version": "1.2.4",
-      "resolved": "https://registry.npmjs.org/@actions/core/-/core-1.2.4.tgz",
-      "integrity": "sha512-YJCEq8BE3CdN8+7HPZ/4DxJjk/OkZV2FFIf+DlZTC/4iBlzYCD5yjRR6eiOS5llO11zbRltIRuKAjMKaWTE6cg=="
+      "version": "1.2.6",
+      "resolved": "https://registry.npmjs.org/@actions/core/-/core-1.2.6.tgz",
+      "integrity": "sha512-ZQYitnqiyBc3D+k7LsgSBmMDVkOVidaagDG7j3fOym77jNunWRuYx7VSHa9GNfFZh+zh61xsCjRj4JxMZlDqTA=="
     },
     "@babel/code-frame": {
       "version": "7.8.3",

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"
   },