Update Swatinem/rust-cache action to v2.8.1

Author: oxide-renovate[bot]

.github/workflows/rust.yml

@@ -37,7 +37,7 @@ jobs:
     - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
       with:
         ref: ${{ github.event.pull_request.head.sha }} # see omicron#4461
-    - uses: Swatinem/rust-cache@98c8021b550208e191a6a3145459bfc9fb29c4c0 # v2.8.0
+    - uses: Swatinem/rust-cache@f13886b937689c021905a6b90929199931d60db1 # v2.8.1
       if: ${{ github.ref != 'refs/heads/main' }}
     - name: Report cargo version
       run: cargo --version
@@ -56,7 +56,7 @@ jobs:
     - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
       with:
         ref: ${{ github.event.pull_request.head.sha }} # see omicron#4461
-    - uses: Swatinem/rust-cache@98c8021b550208e191a6a3145459bfc9fb29c4c0 # v2.8.0
+    - uses: Swatinem/rust-cache@f13886b937689c021905a6b90929199931d60db1 # v2.8.1
       if: ${{ github.ref != 'refs/heads/main' }}
     - name: Build all targets with all features
       uses: actions-rs/cargo@v1