perf: cap CI test workers at 4 to protect shared production host

.gitea/workflows/ci.yaml

@@ -53,9 +53,9 @@ jobs:
           BCRYPT_ROUNDS: 4
         run: |
           if [ -x /usr/bin/time ]; then
-            /usr/bin/time -f 'Tests: %e seconds; peak process RSS: %M KiB' pnpm test:coverage --maxWorkers=6
+            /usr/bin/time -f 'Tests: %e seconds; peak process RSS: %M KiB' pnpm test:coverage --maxWorkers=4
           else
-            time pnpm test:coverage --maxWorkers=6
+            time pnpm test:coverage --maxWorkers=4
           fi
 
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