Avoid host triple computation in BuildParameters.forTriple() (#6906)

We know the triple here, so we shouldn't have to recompute from scratch.

(cherry picked from commit 389edb7)

Author: neonichu

Sources/SPMBuildCore/BuildParameters.swift

@@ -305,10 +305,13 @@ public struct BuildParameters: Encodable {
             testEntryPointPath = nil
         }
 
+        var hostDestination = try Destination.hostDestination()
+        hostDestination.targetTriple = destinationTriple
+
         return try .init(
             dataPath: self.dataPath.parentDirectory.appending(components: ["plugins", "tools"]),
             configuration: self.configuration,
-            toolchain: try UserToolchain(destination: Destination.hostDestination()),
+            toolchain: try UserToolchain(destination: hostDestination),
             hostTriple: self.hostTriple,
             destinationTriple: destinationTriple,
             flags: BuildFlags(),