Auto merge of #294 - striezel-stash:actions-checkout-update, r=jdm

Update actions/checkout in GitHub Actions workflows to v3

Updates the `actions/checkout` action used in the GitHub Actions workflow to its newest major version.

Changes in [actions/checkout](https://github.com/actions/checkout):

> ## v3.1.0
> - Use @actions/core `saveState` and `getState`
> - Add `github-server-url` input
>
> ## v3.0.2
> - Add input `set-safe-directory`
>
> ## v3.0.1
> - Fixed an issue where checkout failed to run in container jobs due to the new git setting `safe.directory`
> - Bumped various npm package versions
>
> ## v3.0.0
>
> - Update to node 16

Still using v2 of `actions/checkout` will generate some warning like in this run: https://github.com/servo/rust-smallvec/actions/runs/3783768281

> Node.js 12 actions are deprecated. For more information see: https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/. Please update the following actions to use Node.js 16: actions/checkout@v2, actions-rs/toolchain@v1

The PR will get rid of those warnings for `actions/checkout`, because v3 uses Node.js 16.

Author: bors-servo

.github/workflows/main.yml

@@ -26,7 +26,7 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Install packages
         if: matrix.os == 'ubuntu-latest'
@@ -98,4 +98,3 @@ jobs:
       - name: Mark the job as unsuccessful
         run: exit 1
         if: "!success()"
-