The environment variable for all desktop platforms is currently
ENABLE_FLUTTER_DESKTOP to allow backward compatibility with existing
workflows. However, this is actually causing issues since 'flutter
config' provides a better experience (most importantly, IDEs will work
correctly, which is generally not the case with the environemnt
variable). Currently people following old (third-patry) instructions
don't realize that they are missing available functionality, so aren't
looking for the current instructions.

In adddition, this will be confusing as different platform graduate to
channels other than master at different times.

To resolve both issues, the environment variable fallback is now
separate for each desktop platform, and follows a naming scheme
consistent with the web environment variable.