[cleanup] Makefile boot-prefix cleanup

[mattpolzin]

Description

This PR does two things:

1. Anywhere that bootstrap-build was hard-coded, I've swapped in ${IDRIS2_BOOT_PREFIX} to avoid unnecessary risk of breakage in the future.
2. I've replaced bootstrap-stage2 ${BOOTSTRAP_PREFIX} with ${IDRIS2_BOOT_PREFIX} as well; this was just another name for the same thing.

This was a fairly straight forward refactor; no intentions of making things behave differently or wiring things up in a different manner (aside from reducing the locations where bootstrap build prefixes needed to be defined).

Should this change go in the CHANGELOG?

• [ ] If this is a fix, user-facing change, a compiler change, or a new paper implementation, I have updated CHANGELOG_NEXT.md (and potentially also CONTRIBUTORS.md).

[mattpolzin]

Heh. Of course I uncovered an unexpected difference between bootstrapping for Windows vs. other platforms.

[mattpolzin]

I think there's something here but I've grown a bit tired of sorting out the problem so I'll close this PR since it was only ever a small deal to me to begin with.