Install deps as part of GitHub Actions workflow

..forgot to install dependencies before executing
`$ npm test` which obviously exploded due to the
fact that `eslint` and friends weren't available.

.github/main.workflow

@@ -3,6 +3,11 @@ workflow "Verify changes on push" {
   on = "push"
 }
 
+action "Install dependencies" {
+  uses = "actions/npm@59b64a598378f31e49cb76f27d6f3312b582f680"
+  args = "install"
+}
+
 action "Run all tests" {
   uses = "actions/npm@59b64a598378f31e49cb76f27d6f3312b582f680"
   args = "test"