Add conformance of an enum to String so that SwiftPM still builds on 5.4. (#3760)

Author: abertelrud

Sources/PackagePlugin/PublicAPI/Context.swift

@@ -105,7 +105,7 @@ public final class TargetBuildContext: Decodable {
     /// Internal
     internal let pluginAction: PluginAction
 
-    internal enum PluginAction: Decodable {
+    internal enum PluginAction: String, Decodable {
         case createBuildToolCommands
     }
 }

Sources/SPMBuildCore/PluginInvocation.swift

@@ -400,7 +400,7 @@ struct PluginScriptRunnerInput: Codable {
         var path: String
     }
     var pluginAction: PluginAction
-    enum PluginAction: Codable {
+    enum PluginAction: String, Codable {
         case createBuildToolCommands
     }
 }