Update actions/checkout action to v6 (#34)

Co-authored-by: oxide-renovate[bot] <146848827+oxide-renovate[bot]@users.noreply.github.com>

Author: oxide-renovate[bot]

.github/workflows/fmt.yml

@@ -7,7 +7,7 @@ jobs:
   check_fmt:
     runs-on: ubuntu-24.04
     steps:
-      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
+      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
       - uses: actions-rs/toolchain@16499b5e05bf2e26879000db0c1d13f7e13fa3af # v1
         with:
           toolchain: stable

.github/workflows/test.yml

@@ -8,7 +8,7 @@ jobs:
     name: cargo test
     runs-on: ubuntu-24.04
     steps:
-      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
+      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
       - uses: actions-rs/toolchain@16499b5e05bf2e26879000db0c1d13f7e13fa3af # v1
         with:
           toolchain: stable