build: use Foundation instead of C for getenv

`getenv` is different across different platforms.  Use Foundation to
abstract over the platform differences.

Author: compnerd

Package.swift

@@ -3,12 +3,6 @@
 import PackageDescription
 import Foundation
 
-#if canImport(Glibc)
-import Glibc
-#else
-import Darwin.C
-#endif
-
 let package = Package(
   name: "SwiftSyntax",
   targets: [
@@ -26,7 +20,7 @@ let swiftSyntaxTarget: PackageDescription.Target
 
 /// If we are in a controlled CI environment, we can use internal compiler flags
 /// to speed up the build or improve it.
-if getenv("SWIFT_BUILD_SCRIPT_ENVIRONMENT") != nil {
+if ProcessInfo.processInfo.environment("SWIFT_BUILD_SCRIPT_ENVIRONMENT") != nil {
   let groupFile = URL(fileURLWithPath: #file)
     .deletingLastPathComponent()
     .appendingPathComponent("utils")