Bail if plugin-files is set after plugins have been loaded.

We know the flag will be ignored but the user wants it to take effect.

src/libstore/globals.cc

@@ -243,6 +243,14 @@ void MaxBuildJobsSetting::set(const std::string & str, bool append)
 }
 
 
+void PluginFilesSetting::set(const std::string & str, bool append)
+{
+    if (pluginsLoaded)
+        throw UsageError("plugin-files set after plugins were loaded, you may need to move the flag before the subcommand");
+    BaseSetting<Paths>::set(str, append);
+}
+
+
 void initPlugins()
 {
     for (const auto & pluginFile : settings.pluginFiles.get()) {
@@ -270,6 +278,9 @@ void initPlugins()
        unknown settings. */
     globalConfig.reapplyUnknownSettings();
     globalConfig.warnUnknownSettings();
+
+    /* Tell the user if they try to set plugin-files after we've already loaded */
+    settings.pluginFiles.pluginsLoaded = true;
 }
 
 }

src/libstore/globals.hh

@@ -28,6 +28,23 @@ struct MaxBuildJobsSetting : public BaseSetting<unsigned int>
     void set(const std::string & str, bool append = false) override;
 };
 
+struct PluginFilesSetting : public BaseSetting<Paths>
+{
+    bool pluginsLoaded = false;
+
+    PluginFilesSetting(Config * options,
+        const Paths & def,
+        const std::string & name,
+        const std::string & description,
+        const std::set<std::string> & aliases = {})
+        : BaseSetting<Paths>(def, name, description, aliases)
+    {
+        options->addSetting(this);
+    }
+
+    void set(const std::string & str, bool append = false) override;
+};
+
 class Settings : public Config {
 
     unsigned int getDefaultCores();
@@ -819,7 +836,7 @@ public:
     Setting<uint64_t> minFreeCheckInterval{this, 5, "min-free-check-interval",
         "Number of seconds between checking free disk space."};
 
-    Setting<Paths> pluginFiles{
+    PluginFilesSetting pluginFiles{
         this, {}, "plugin-files",
         R"(
           A list of plugin files to be loaded by Nix. Each of these files will