Bump actions/core version

Fixed "The `set-env` command is disabled"

sugestion from golangci#103 (comment)

Author: sternik

package.json

@@ -26,7 +26,7 @@
   "license": "MIT",
   "dependencies": {
     "@actions/cache": "^0.2.1",
-    "@actions/core": "^1.2.0",
+    "@actions/core": "^1.2.6",
     "@actions/exec": "^1.0.1",
     "@actions/github": "^2.1.1",
     "@actions/tool-cache": "^1.5.5",