Add back propagating '-external-plugin-path' driver agruments

Author: rintaro

Sources/SwiftDriver/Jobs/FrontendJobHelpers.swift

@@ -275,6 +275,10 @@ extension Driver {
       commandLine.appendPath(localPluginPath)
     }
 
+    if isFrontendArgSupported(.externalPluginPath) {
+      try commandLine.appendAll(.externalPluginPath, from: &parsedOptions)
+    }
+
     if isFrontendArgSupported(.blockListFile) {
       try Driver.findBlocklists(RelativeTo: try toolchain.executableDir).forEach {
         commandLine.appendFlag(.blockListFile)