don't install jj via brew if it's here

install_extras.py

@@ -241,6 +241,10 @@ def git_whence():
 	os.chmod(CURRENT_DIR / 'git-whence', 0o755)
 
 def jujutsu():
+	if (CURRENT_DIR / 'jj').exists():
+		print_gray('jj already downloaded')
+		return
+
 	if platform.system() == 'Darwin':
 		print('$ brew install jj')
 		subprocess.run(['brew', 'install', 'jj'], check=True)
@@ -248,10 +252,6 @@ def jujutsu():
 	else:
 		assert platform.system() == 'Linux'
 
-	if (CURRENT_DIR / 'jj').exists():
-		print_gray('jj already downloaded')
-		return
-
 	client = httpx.Client()
 	latest = gh_latest_version(client, 'jj-vcs', 'jj')
 	version = latest['name']