Fix finding of xz in the PATH

 - Seems like the environment needed to be passed to the which() method.

Sources/SwiftSDKGenerator/Generator/SwiftSDKGenerator+Download.swift

@@ -86,7 +86,7 @@ extension SwiftSDKGenerator {
     let xzPath = try await which("xz")
     if xzPath == nil {
       logger.warning("""
-      The 'xz' utility was not found in path. \
+      The `xz` utility was not found in `PATH`. \
       Consider installing it for more efficient downloading of package lists.
       """)
     }

Sources/SwiftSDKGenerator/SystemUtils/which.swift

@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 import AsyncProcess
+import Foundation
 
 /// Look for an executable using the `which` utility.
 /// 
@@ -19,9 +20,12 @@ import AsyncProcess
 /// - Returns: The path to the executable if found, otherwise nil.
 func which(_ executableName: String) async throws -> String? {
   let result = try await ProcessExecutor.runCollectingOutput(
-    executable: "/usr/bin/which", [executableName], collectStandardOutput: true, collectStandardError: false
+    executable: "/usr/bin/which", [executableName], collectStandardOutput: true, collectStandardError: false,
+    environment: ProcessInfo.processInfo.environment
   )
 
+  try result.exitReason.throwIfNonZero()
+
   if let output = result.standardOutput {
     let path = String(buffer: output).trimmingCharacters(in: .whitespacesAndNewlines)
     return path.isEmpty ? nil : path