Seed ssh known hosts with github's public key (facebook#28370)

Summary:
The [previous attempt](facebook#28304) to fix the publish step failed, so now reverting to manually configuring things. This PR adds an entry to SSH’s `known_hosts` file using github.com’s public key that I have verified as per [these instructions](https://serverfault.com/a/807363):

```
~/C/R/react-native [master] » nmap github.com --script ssh-hostkey
Nmap scan report for github.com (140.82.118.4)
rDNS record for 140.82.118.4: lb-140-82-118-4-ams.github.com
PORT     STATE SERVICE
22/tcp   open  ssh
| ssh-hostkey:
|   1024 ad:1c:08:a4:40:e3:6f:9c:f5:66:26:5d:4b:33:5d:8c (DSA)
|_  2048 16:27:ac:a5:76:28:2d:36:63:1b:56:4d:eb:df:a6:48 (RSA)
```

These fingerprints line up with [the ones posted by GitHub](https://help.github.com/en/github/authenticating-to-github/githubs-ssh-key-fingerprints), so my setup should be good and can be trusted to grab the public key from the right host:

```
~/C/R/react-native [master] » ssh-keyscan -t rsa -H github.com
|1|If6MU203eXTaaWL678YEfWkVMrw=|kqLeIAyTy8pzpj8x8Ae4Fr8Mtlc= ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAq2A7hRGmdnm9tUDbO9IDSwBK6TbQa+PXYPCPy6rbTrTtw7PHkccKrpp0yVhp5HdEIcKr6pLlVDBfOLX9QUsyCOV0wzfjIJNlGEYsdlLJizHhbn2mUjvSAHQqZETYP81eFzLQNnPHt4EVVUh7VfDESU84KezmD5QlWpXLmvU31/yMf+Se8xhHTvKSCZIFImWwoG6mbUoWf9nzpIoaSjB+weqqUUmpaaasXVal72J+UX2B+2RPW3RcT0eOzQgqlJL3RKrTJvdsjE3JEAvGq3lGHSZXy28G3skua2SmVi/w4yCE6gbODqnTWlg7+wC604ydGXA8VJiS5ap43JXiUFFAaQ==
```

[Internal] [Fixed] - Make automated publishing of packages from CI work again
Pull Request resolved: facebook#28370

Test Plan: I used the command being added in this PR in [a failed CI job](https://app.circleci.com/pipelines/github/facebook/react-native/4104/workflows/916127cb-177f-4583-9f90-cae5318041d8/jobs/140810). When I invoked the publish script manually I was not greeted by the blocking prompt and the package was successfully published: https://www.npmjs.com/package/react-native/v/0.0.0-56cf99a96

Reviewed By: cpojer

Differential Revision: D20601527

Pulled By: hramos

fbshipit-source-id: b1a4405228408cfc4a1b3b44ab88c79522af3a66

.circleci/config.yml

@@ -568,11 +568,15 @@ jobs:
   # -------------------------
   #      JOBS: Releases
   # -------------------------
-  # Publishes new version onto npm
-  # Only works on stable branches when a properly tagged commit is pushed
+  # Publishes a new version onto npm
   publish_npm_package:
     executor: reactnativeandroid
     steps:
+      - run:
+          name: Add github.com to SSH known hosts
+          command: |
+            mkdir -p ~/.ssh
+            echo '|1|If6MU203eXTaaWL678YEfWkVMrw=|kqLeIAyTy8pzpj8x8Ae4Fr8Mtlc= ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAq2A7hRGmdnm9tUDbO9IDSwBK6TbQa+PXYPCPy6rbTrTtw7PHkccKrpp0yVhp5HdEIcKr6pLlVDBfOLX9QUsyCOV0wzfjIJNlGEYsdlLJizHhbn2mUjvSAHQqZETYP81eFzLQNnPHt4EVVUh7VfDESU84KezmD5QlWpXLmvU31/yMf+Se8xhHTvKSCZIFImWwoG6mbUoWf9nzpIoaSjB+weqqUUmpaaasXVal72J+UX2B+2RPW3RcT0eOzQgqlJL3RKrTJvdsjE3JEAvGq3lGHSZXy28G3skua2SmVi/w4yCE6gbODqnTWlg7+wC604ydGXA8VJiS5ap43JXiUFFAaQ==' >> ~/.ssh/known_hosts
       - restore_cache_checkout:
           checkout_type: android
       - run_yarn