devex: performance monitor environment variable

Instead of requiring developers to keep changes in their git index to
use the performance monitor, make it configurable through the .env
file.

tools/run-selfhosted.js

@@ -98,6 +98,7 @@ const main = async () => {
         InternetModule,
         DevelopmentModule,
         DNSModule,
+        PerfMonModule,
     } = (await import('@heyputer/backend')).default;
 
     const k = new Kernel({
@@ -116,6 +117,9 @@ const main = async () => {
     k.add_module(new PuterAIModule());
     k.add_module(new InternetModule());
     k.add_module(new DNSModule());
+    if ( process.env.PERFMON ) {
+        k.add_module(new PerfMonModule());
+    }
     if ( process.env.UNSAFE_PUTER_DEV ) {
         k.add_module(new DevelopmentModule());
     }