add nightly swift 5.4 docker to CI

Author: mattpolzin

.github/workflows/tests.yml

@@ -23,6 +23,7 @@ jobs:
           - swift:5.3-focal
           - swift:5.3-centos8
           - swift:5.3-amazonlinux2
+          - swiftlang/swift:nightly-5.4-focal
     container: ${{ matrix.image }}
     steps:
     - name: Checkout code