Thanks Aditya.

What do you think of changing the default luatex-cache directory to the
system's temporary directory? Consider:

   - The $HOME directory is sacrosanct (4784 people agree:
   https://bugs.launchpad.net/ubuntu/+source/snapd/+bug/1575053)
   - The temp directory is cleared on Linux (Unix?) system reboots; purged
   during regular Windows upkeep
   - The temp directory is writable by default
   - Changing the location requires calling an additional program, which
   isn't obvious (principle of least astonishment)

My text editor invokes ConTeXt like:

    if( TYPESETTER.canRun() ) {
      env.put( "TEXMFCACHE", System.getProperty( "java.io.tmpdir" ) );

      mArgs.add( TYPESETTER.getName() );
      mArgs.add( .. --path .. --purge .. --batch .. --result ..
--environment .. etc. );
      mArgs.add( inputFilename );
    }

The first line ensures that "context" is an executable located in a PATH
directory. The second line attempts to change the luatex-cache directory.
The remaining lines configure the command-line arguments prior to running
ConTeXt.

Fearing flaming wrath from users, an additional mtxrun call is required,
which incurs overhead:

   - Check for mtxrun executable
   - Run mtxrun each time

This would work but feels like a leaky abstraction (i.e., the context
executable should honour TEXMFCACHE without needing to invoke mtxrun
because context creates the luatex-cache directory).

Thoughts?