Invalidate image cache on git clones

[JoshCu]

Adds lines to the dockerfile that make sure the cache is not used if remote repositories of ngen and troute have been updated

It's something I've had in my fork for a bit and thought could be useful. It means we can build the images without --no-cache and not have to worry about it using an old version of troute or ngen

[benlee0423]

@JoshCu
The build on x86 platform took long time to finish. It usually finishes about 10 or 11 min, and it didn't finish more than 29 min as of this writing.
Might be this change causing this delay?

[JoshCu]

Those two lines will invalidate any existing build cache we have for layers after them so the first time might take longer but then subsequent builds should be able to do without clearing the build cache and they should be much quicker.
I added it to my fork to let me use build cache with docker build cloud but it should work locally or on selfhosted instances with persistent storage too.
The build cloud CI ends up being reasonably simple https://github.com/JoshCu/NGIAB-CloudInfra/blob/new_ci-test/.github/workflows/build-and-push.yml and the repeated builds are much quicker.

Although now I'm having issues where the version of python ngen is built with is cached and the version of python in the final image isn't which is breaking ngen.

If I can get it to work it should speed up repeat builds a lot, but right now it isn't :(