Update GitHub Actions actions/checkout@v2 to v3

The v2 implementation uses Node 12, which is end-of-life on April 30, 2022.
See https://nodejs.org/en/about/releases/. Update to v3, which is based on
Node 16 whose support lasts until April 30, 2024.

.github/workflows/rust.yml

@@ -11,23 +11,23 @@ jobs:
   miri:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - env:
         TARGET: x86_64-unknown-linux-gnu
       run: sh ci/miri.sh
 
   rustfmt_clippy:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - env:
         TARGET: i586-unknown-linux-gnu
       run: sh ci/tools.sh
 
   test:
     runs-on: ${{ matrix.os }}
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - uses: actions-rs/toolchain@v1
       with:
         toolchain: ${{ matrix.channel }}