chore(bun): Fix install-bun.js version check and improve upgrade feedback (#18492)

1. fixed incorrect Bun version command
2. removed unused variable
3. improved user feedback(logs a clear message when Bun is already on
the latest version instead of silently exiting)

---------

Co-authored-by: Lukas Stracke <lukas.stracke@sentry.io>

Author: Mohataseem89

packages/bun/scripts/install-bun.js

@@ -10,13 +10,11 @@ const https = require('https');
 const installScriptUrl = 'https://bun.sh/install';
 
 // Check if bun is installed
-exec('bun -version', (error, version) => {
+exec('bun --version', (error, version) => {
   if (error) {
     console.error('bun is not installed. Installing...');
     installLatestBun();
   } else {
-    const versionBefore = version.trim();
-
     exec('bun upgrade', (error, stdout, stderr) => {
       if (error) {
         console.error('Failed to upgrade bun:', error);
@@ -26,6 +24,7 @@ exec('bun -version', (error, version) => {
       const out = [stdout, stderr].join('\n');
 
       if (out.includes("You're already on the latest version of Bun")) {
+        console.log('Bun is already up to date.');
         return;
       }