FLUTTER_VERSION variable do not work, throws error every time

[danielgomezrico]

I found that this pack always try to build the app with the latest flutter version available, tho the docs says that if you set the variable FLUTTER_VERSION it should install it, but it throws an error, out example:

-----> Restoring Flutter SDK from CACHE
Building flutter tool...
-----> Installing Flutter SDK version : 3.13.2
Could not find a command named "version".
Run 'flutter -h' (or 'flutter <command> -h') for available flutter commands and options.

Related to:

• In current version of flutter(2.8.1) not working #13
• How to switch to stable release? #12

Idea

The way to install new version of flutter is not using flutter version because that command do not exist, it is bad here:
https://github.com/diezep/heroku-buildpack-flutter/blob/eecebf9581129aa31f587cdf41ec2a3cbf0fdbe4/bin/compile#L48C11-L48C26