Update pr auditor job to Go 1.8

Author: olafurpg

.github/workflows/pr-auditor.yml

@@ -10,7 +10,7 @@ jobs:
       - uses: actions/checkout@v2
         with: { repository: 'sourcegraph/sourcegraph' }
       - uses: actions/setup-go@v2
-        with: { go-version: '1.17' }
+        with: { go-version: '1.18' }
 
       - run: ./dev/pr-auditor/check-pr.sh
         env: