Whatever compiler you are using (Arduino IDE?) should have a 'verbose' setting that you can activate. This will give you a HUGE amount of output, but will also show you exactly what file is being accessed for the library in question.
When I do this, I copy/paste the entire output into Notepad++ and use its search capabilities to sort through all the chaff.
Looks like you have two choices
Use a different controller that is compatible with the normal Arduino Wire library - maybe something like the Nano?
Modify SparkFunCCS811.h to #include TinyWireM.h instead of Wire.h. This is actually not hard at all - just takes a bit of patience and the knowledge that "there is no magic".
@Majenko pointed me to the root cause of the problem: I was sending MIDI notes via Ableton's feature of using the computer's keyboard. It seems that the OS (or the keyboard) cannot send/receive more than six simultaneous keystrokes.