fix: add branch export for rolling installation (#3054)

README.md

@@ -42,7 +42,7 @@ bash <(curl -s https://raw.githubusercontent.com/lunarvim/lunarvim/master/utils/
 If you are running Neovim 0.8+
 
 ```bash
-bash <(curl -s https://raw.githubusercontent.com/lunarvim/lunarvim/rolling/utils/installer/install.sh)
+export LV_BRANCH="rolling"; bash <(curl -s https://raw.githubusercontent.com/lunarvim/lunarvim/rolling/utils/installer/install.sh)
 ```
 
 To run the install script without any interaction you can pass the `-y` flag to automatically install all dependencies and have no prompts. This is particularly useful in automated installations.