move to stable 5.4 docker images for testing.

Author: mattpolzin

.github/workflows/tests.yml

@@ -25,7 +25,11 @@ jobs:
           - swift:5.3-focal
           - swift:5.3-centos8
           - swift:5.3-amazonlinux2
-          - swiftlang/swift:nightly-5.4-focal
+          - swift:5.4-xenial
+          - swift:5.4-bionic
+          - swift:5.4-focal
+          - swift:5.4-centos8
+          - swift:5.4-amazonlinux2
     container: ${{ matrix.image }}
     steps:
     - name: Checkout code