Add nightly build (#28066)

Summary:
Note: I'm currently testing this, so do not review just yet.

Adding a nightly CI build in preparation for further work on [React Native: Benchmark Suite](react-native-community/discussions-and-proposals#186.)

Currently it will only run the job `nightly_job` which will simply run `$ echo "Nightly build run"`. The plan is to do nightly checks and performance status on the React Native repo, and create a dashboard where we can see performance changes over time. More on this in the proposal: [React Native: Benchmark Suite](react-native-community/discussions-and-proposals#186.)

## Changelog

[General] [Added] - Add nightly CI build
Pull Request resolved: #28066

Test Plan: Individual jobs will have their own test plan.

Reviewed By: sebmck

Differential Revision: D20008434

Pulled By: hramos

fbshipit-source-id: a5e8aad616c28bfad8f3455b5ebadf7a7d174a55

.circleci/config.yml

@@ -617,10 +617,23 @@ jobs:
           echo "machine github.com login react-native-bot password $GITHUB_TOKEN" > ~/.netrc
       - run: node ./scripts/publish-npm.js
 
+  # -------------------------
+  #    JOBS: Nightly
+  # -------------------------
+  nightly_job:
+    machine: true
+    steps:
+      - run:
+          name: Nightly
+          command: |
+            echo "Nightly build run"
+
 # -------------------------
 #        WORK FLOWS
 # -------------------------
 workflows:
+  version: 2
+
   tests:
     jobs:
       - setup:
@@ -722,3 +735,14 @@ workflows:
       - js_coverage:
           requires:
             - setup
+
+  nightly:
+    triggers:
+      - schedule:
+          cron: "0 * * * *"
+          filters:
+            branches:
+              only:
+                - master
+    jobs:
+      - nightly_job