fix(ci): execution of Cypress test on GitHub Actions workflow (#274)

To fix failing CI workflow: https://github.com/openwhyd/openwhyd/runs/477958067

Initially, I thought that some dependencies were missing.

Actually, downgrading the `ubuntu` images fixed the issue.

Possible next step: simplify `nodejs.yml` by using `cypress-io/github-action@v1` instead of setting up everything ourselves, as explained in https://github.com/cypress-io/github-action#basic.

.github/workflows/nodejs.yml

@@ -69,7 +69,7 @@ jobs:
   cypress:
     name: Cypress E2E (experimental)
     if: "!contains(github.event.head_commit.message, '[skip ci]')"
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-16.04
     services:
       mongodb:
         image: docker://mongo:3.4