Fix GitHub Actions secrets access (facebook#1005)

Summary:
Ref: [Secrets documentation](https://help.github.com/en/actions/configuring-and-managing-workflows/creating-and-storing-encrypted-secrets#using-encrypted-secrets-in-a-workflow)

In combination with setting the secrets in the GitHub settings, this should fully migrate yoga to GH actions.

Pull Request resolved: facebook#1005

Test Plan: GitHub Actions CI on the PR attached to this diff

Reviewed By: SidharthGuglani

Differential Revision: D21574995

Pulled By: bigfootjon

fbshipit-source-id: e9ee6d7cf1ae131afdc3aa3778667f1e9b9de833

.github/workflows/ci.yml

@@ -34,8 +34,8 @@ jobs:
       - uses: actions/checkout@v2
       - name: Install dependencies
         run: |
-          if [[ -n "$encrypted_d27e803291ff_iv" ]]; then
-            openssl aes-256-cbc -K $encrypted_d27e803291ff_key -iv $encrypted_d27e803291ff_iv -in scripts/setup-keys.enc -d >> gradle.properties;
+          if [[ -n "${{ secrets.encrypted_d27e803291ff_iv }}" ]]; then
+            openssl aes-256-cbc -K ${{ secrets.encrypted_d27e803291ff_key }} -iv {{ secrets.encrypted_d27e803291ff_iv }} -in scripts/setup-keys.enc -d >> gradle.properties;
           fi
           sudo apt-get update
           sudo apt-get install -y ninja-build