Merge pull request #908 from Quick/ci-swift-5.5-development

[CI] Test Swift 5.5 Development

Author: ikesyo

.github/workflows/ci-swiftpm.yml

@@ -31,6 +31,7 @@ jobs:
         container:
           - swift:5.3
           - swift:5.4
+          - swiftlang/swift:nightly-5.5
           - swiftlang/swift:nightly
       fail-fast: false
     container: ${{ matrix.container }}