Add explicit dependency on @actions/exec

Author: eregon

package.json

@@ -24,6 +24,7 @@
   "homepage": "https://github.com/eregon/use-ruby-action",
   "dependencies": {
     "@actions/core": "^1.1.1",
+    "@actions/exec": "^1.0.1",
     "@actions/io": "^1.0.1",
     "@actions/tool-cache": "^1.1.2",
     "axios": "^0.19.0"