Allow a custom env for more xcrun invocations

Basically a continuation of #1766

Author: neonichu

Sources/Workspace/UserToolchain.swift

@@ -129,6 +129,8 @@ public final class UserToolchain: Toolchain {
         return lookupExecutablePath(filename: getenv(variable), searchPaths: searchPaths)
     }
 
+    private let environment: [String:String]
+
     public func getClangCompiler() throws -> AbsolutePath {
 
         if let clangCompiler = _clangCompiler {
@@ -142,7 +144,7 @@ public final class UserToolchain: Toolchain {
             clangCompiler = value
         } else {
             // No value in env, so search for `clang`.
-            let foundPath = try Process.checkNonZeroExit(arguments: whichClangArgs).chomp()
+            let foundPath = try Process.checkNonZeroExit(arguments: whichClangArgs, environment: environment).chomp()
             guard !foundPath.isEmpty else {
                 throw InvalidToolchainDiagnostic("could not find `clang`")
             }
@@ -159,6 +161,7 @@ public final class UserToolchain: Toolchain {
 
     public init(destination: Destination, environment: [String:String] = Process.env) throws {
         self.destination = destination
+        self.environment = environment
 
         // Get the search paths from PATH.
         let searchPaths = getEnvSearchPaths(