Option to disable precompilation fallback

[LLFourn]

I've had a couple of times where I released a module which breaks pre-compilation.
It hits one of the precomp bugs, fails to compile, falls back to compiling the original source, but I don't notice it because prove doesn't show it. I want the ability to disable precomp fallback, so it just dies completely.

I think it might be good idea to disable it for make spectest too and see what comes up.

An environment variable would be handy but I guess a pragma like no precomp-fallback could work too.

I think I am talking wrapping this line in an if statement: https://github.com/rakudo/rakudo/blob/nom/src/core/CompUnit/Repository/FileSystem.pm#L78

Thoughts?

EDIT note: I think I should mention that in general precomp is working great so (@niner)++. It has cut one of my project's startup time from around 40s to 1s. I love it so much that I want to make sure that my changes don't quietly break it!