GHActions: Don't shallow clone submodules

[TellowKrinkle]

Description of Changes

Shallow clones tend to be 5-10s slower

GHActions Windows submodule checkout run time (seconds):

Linux:

macOS (never used GH cache on submodules):

Rationale behind Changes

Why waste 10 seconds when you don't have to?

Suggested Testing Steps

Watch the CI run

[TellowKrinkle]

We can also do this by removing the shallow mark from .gitmodules, depending on whether we want to change the default for non-CI users checking out the repo. Preferences?

[xTVaser]

this is really counter-intuitive. Everything i read suggests that shallow clones are intuitively the right choice for temp CI builds, but also faster. But I can't argue with the graphs :/