update Swift version to 5.8 on GHA (#7)

Author: MaximBazarov

.github/workflows/swift-build-test.yml

@@ -18,7 +18,7 @@ jobs:
     steps:
     - uses: swift-actions/setup-swift@v1
       with:
-        swift-version: "5.7"
+        swift-version: "5.8"
     - uses: actions/checkout@v3
     - name: Test
       run: swift test -v