Bump actions/checkout from 2 to 3

Bumps [actions/checkout](https://github.com/actions/checkout) from 2 to 3.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v2...v3)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/nix-flake-check.yml

@@ -6,7 +6,7 @@ jobs:
   tests:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: cachix/install-nix-action@v17
         with:
           install_url: https://github.com/numtide/nix-flakes-installer/releases/download/nix-2.4pre20210604_8e6ee1b/install

.github/workflows/nix-flake-update.yml

@@ -8,7 +8,7 @@ jobs:
   createPullRequest:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: cachix/install-nix-action@v17
         with:
           install_url: https://github.com/numtide/nix-flakes-installer/releases/download/nix-2.4pre20210604_8e6ee1b/install