If the script ends with an AppleScript extension (like .scpt) then it will be passed to osascript.
Otherwise if it is executable, it will be passed to the shell to execute.
Note that you can also just save a JXA script as a .scpt and that will work as well because .scpt files include the language. So another possibility would be this .scpt:
var kme = Application("Keyboard Maestro Engine");
var varName = ObjC.unwrap($.NSProcessInfo.processInfo.environment.objectForKey("KMPARAM_Variable_name"))
var varValue = kme.getvariable( varName );
Saved as a script called Action.scpt from AppleScript and referenced in your plist.
You might like to try that too.
The AppleScript .scpt is pre-compiled.
That might make a difference. But this is a relatively trivial script, so all the time will be in the startup.
Although note that the AppleScript gets the variable name via system attribute, which will not work properly for non-ASCII variable names, where the JXA I wrote gets it from the environment dictionary, which may be unicode-safe (I'm not sure).