Update actions/checkout action to v6

Author: renovate[bot]

.github/workflows/build.yml

@@ -16,7 +16,7 @@ jobs:
 
     steps:
       # check out our code
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       # install rust toolchain
       - uses: actions-rs/toolchain@v1

.github/workflows/formatting.yml

@@ -10,7 +10,7 @@ jobs:
     permissions:
       contents: read
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - uses: dtolnay/rust-toolchain@nightly
         with:
           components: rustfmt