[6.0.0][PackageModel] Inject swift-testing flags only if toolchain is target… (#7922)

- Explanation:

Fixes a bug where swift and linker flags for swift-testing were injected
into `extraFlags` of a toolchain that was targeting WASM but used macOS
to build.

- Main Branch PR:
#7920
- 6.0 Branch PR:
#7921

- Resolves:
#7919
- Resolves: rdar://134714404

- Risk: Low (only affects cross-compilation to WASM).

- Reviewed By: @bnbarham 

- Testing: Manual testing since this is not currently possible to test
automatically.

Author: xedin

Sources/PackageModel/UserToolchain.swift

@@ -660,14 +660,15 @@ public final class UserToolchain: Toolchain {
         var swiftCompilerFlags: [String] = []
         var extraLinkerFlags: [String] = []
 
-        #if os(macOS)
-        let (swiftCFlags, linkerFlags) = Self.deriveMacOSSpecificSwiftTestingFlags(
-            derivedSwiftCompiler: swiftCompilers.compile,
-            fileSystem: fileSystem
-        )
-        swiftCompilerFlags += swiftCFlags
-        extraLinkerFlags += linkerFlags
-        #endif
+        if triple.isMacOSX {
+            let (swiftCFlags, linkerFlags) = Self.deriveMacOSSpecificSwiftTestingFlags(
+                derivedSwiftCompiler: swiftCompilers.compile,
+                fileSystem: fileSystem
+            )
+
+            swiftCompilerFlags += swiftCFlags
+            extraLinkerFlags += linkerFlags
+        }
 
         swiftCompilerFlags += try Self.deriveSwiftCFlags(
             triple: triple,