Remove warnings as errors from 5.9 main runs (#245)

See #243

Author: gjcairo

.github/workflows/main.yml

@@ -11,7 +11,7 @@ jobs:
     name: Unit tests
     uses: apple/swift-nio/.github/workflows/unit_tests.yml@main
     with:
-      linux_5_9_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
+      linux_5_9_arguments_override: "--explicit-target-dependency-import-check error"
       linux_5_10_arguments_override: "--explicit-target-dependency-import-check error"
       linux_6_0_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
       linux_nightly_next_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"