fix process env

Sources/CartonHelpers/Basics/Process/ProcessEnv.swift

@@ -10,11 +10,13 @@
 
 import Foundation
 
-public struct ProcessEnvironmentKey {
+public struct ProcessEnvironmentKey: CustomStringConvertible {
   public let value: String
   public init(_ value: String) {
     self.value = value
   }
+
+  public var description: String { value }
 }
 
 extension ProcessEnvironmentKey: Encodable {

Tests/CartonTests/ProcessTests.swift

@@ -0,0 +1,16 @@
+import XCTest
+import CartonHelpers
+
+final class ProcessTests: XCTestCase {
+  func testProcessEnv() async throws {
+    let proc = Process(
+      arguments: ["/usr/bin/env"],
+      environmentBlock: ["PATH": "/usr/local/bin:/usr/bin"]
+    )
+    try proc.launch()
+    let result = try await proc.waitUntilExit()
+    let out = try result.utf8Output()
+      .trimmingCharacters(in: .whitespacesAndNewlines)
+    XCTAssertEqual(out, "PATH=/usr/local/bin:/usr/bin")
+  }
+}