Change packer GitHub action (#1567)

Summary:
The one we're using right now appears to be borked: https://github.com/facebook/flipper/runs/1200221471

Pull Request resolved: https://github.com/facebook/flipper/pull/1567

Test Plan: Watching the CI here.

Reviewed By: mweststrate

Differential Revision: D24110022

Pulled By: passy

fbshipit-source-id: 6b9159ab634417123dc9512b293bbce77faff0f4

.github/workflows/packer.yml

@@ -7,18 +7,16 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
+      - name: Setup toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          toolchain: stable
       - name: Test
-        uses: icepuma/rust-action@1.44.0
-        with:
-          args: cd packer && cargo test
+        run: cd packer && cargo test
       - name: Format
-        uses: icepuma/rust-action@1.44.0
+        run: cd packer && cargo fmt -- --check
         continue-on-error: true
-        with:
-          args: cd packer && cargo fmt -- --check
       - name: Lint
-        uses: icepuma/rust-action@1.44.0
+        run: cd packer && cargo clippy -- -Dwarnings
         continue-on-error: true
-        with:
-          args: cd packer && cargo clippy -- -Dwarnings