Update CI to actions/checkout@v3 (#40)

Author: johnthagen

.github/workflows/build.yml

@@ -11,7 +11,7 @@ jobs:
         toolchain: [ stable, 1.28.0, nightly ]
     runs-on: ${{ matrix.platform }}
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: actions-rs/toolchain@v1
         with:
           toolchain: ${{ matrix.toolchain }}
@@ -28,7 +28,7 @@ jobs:
         toolchain: [ stable, 1.30.0, nightly ]
     runs-on: ${{ matrix.platform }}
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: actions-rs/toolchain@v1
         with:
           toolchain: ${{ matrix.toolchain }}
@@ -43,7 +43,7 @@ jobs:
         project_dir: [ build_std, no_main ]
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: actions-rs/toolchain@v1
         with:
           toolchain: nightly