ci: Disable explicit target dependency import checks for swift versions before Swift 6, due to associated false positive errors. See FB14859516.

Author: scottmarchant

.github/workflows/pull_request.yml

@@ -17,8 +17,24 @@ jobs:
     name: Unit tests
     uses: apple/swift-nio/.github/workflows/unit_tests.yml@main
     with:
-      linux_5_9_arguments_override: "--explicit-target-dependency-import-check error"
-      linux_5_10_arguments_override: "--explicit-target-dependency-import-check error"
+      
+      # NOTE: Aug 5, 2025. We've determined SPM has bugs for `condition: .when(platforms: [.somePlatform])`
+      # that incorrectly ignore the condition and import anyways for unintended platforms.
+      #
+      # See https://github.com/apple/swift-distributed-tracing/actions/runs/16578688393/job/46920025183?pr=174
+      # for an example of the resulting build failure.
+      #
+      # This is documented as a bug in Swift 5.10, resolved in Swift 6.
+      # See FB14859516.
+      # See also https://forums.swift.org/t/do-swiftpm-conditional-target-dependencies-work/74047
+      #
+      # Given this, the explicit target dependency check is disabled for Swift versions before Swift 6.
+
+      # Intentionally disabled:
+      #
+      # 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: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
       linux_6_1_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
       linux_nightly_6_0_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"