Set up GitHub test workflow for Packer (#1306)

Summary:
Per title.
Pull Request resolved: https://github.com/facebook/flipper/pull/1306

Test Plan: CI

Reviewed By: mweststrate

Differential Revision: D22285900

Pulled By: passy

fbshipit-source-id: 094cab21cc0f646495e9166435fb6563d50ee2a8

.github/workflows/packer.yml

@@ -0,0 +1,24 @@
+name: Packer
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v1
+      - name: Test
+        uses: icepuma/rust-action@1.44.0
+        with:
+          args: cd packer && cargo test
+      - name: Format
+        uses: icepuma/rust-action@1.44.0
+        continue-on-error: true
+        with:
+          args: cd packer && cargo fmt -- --check
+      - name: Lint
+        uses: icepuma/rust-action@1.44.0
+        continue-on-error: true
+        with:
+          args: cd packer && cargo clippy -- -Dwarnings