I couldn’t find this in the docs anywhere, but I did find it mentioned in some old emails. You can use environment variables in the .wkg file, like this:

$SITOA_WKG_PATH
$MOOTZOID_WKG_PATH

or this:

${SITOA_WKG_PATH}
${MOOTZOID_WKG_PATH}

Either syntax worked for me when I tried it.

There does seem to be some weirdness with loading workgroup shaders this way, but that’s still under investigation.