fix #26207 .gitignore only include project root build directory (#26388)

packages/flutter_tools/templates/app/.gitignore.tmpl

diff --git a/packages/flutter_tools/templates/app/.gitignore.tmpl b/packages/flutter_tools/templates/app/.gitignore.tmpl
index 9d7edcf..07488ba 100644
--- a/packages/flutter_tools/templates/app/.gitignore.tmpl
+++ b/packages/flutter_tools/templates/app/.gitignore.tmpl
@@ -25,7 +25,7 @@
 .packages
 .pub-cache/
 .pub/
-build/
+/build/
 
 # Android related
 **/android/**/gradle-wrapper.jar