chore(deps): update actions/cache action to v5

Generated by renovateBot

Author: renovate[bot]

.github/workflows/buildtest.yaml

@@ -26,7 +26,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6
-      - uses: actions/cache@v4
+      - uses: actions/cache@v5
         with:
           path: |
             ~/go/pkg/mod