add swift 5.4 as a build target

Author: samisuteria

.github/workflows/build.yml

@@ -76,7 +76,7 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        swift: ["5.5", "5.6"]
+        swift: ["5.4", "5.5", "5.6"]
     steps:
     - uses: swift-actions/setup-swift@v1
       with: