ci: Lock PostgreSQL database creation (#94)

There have been race conditions when multiple instances
are created at once. This is an attempt to fix!

.github/workflows/coder.yaml

@@ -159,7 +159,7 @@ jobs:
         run:
           DB=true gotestsum --jsonfile="gotests.json" --packages="./..." --
           -covermode=atomic -coverprofile="gotests.coverage" -timeout=3m
-          -count=1 -race -parallel=2
+          -count=1 -race -parallel=1
 
       - uses: codecov/codecov-action@v2
         with: