Removing need for FSTAR_HOME

[mtzguido]

(from commit messages:)

• F* discovery: use FSTAR_EXE ?= fstar.exe
  Note: 3d itself also inspects this variable (it was inspecting
  FSTAR_HOME before). It may be easier to remove this behavior and add a
  flag, so the fstar used is explicit in the Makefiles.

• Remove most uses of FSTAR_HOME
  A few remain in package.sh, but there it does in fact need a repo
  (afaict) so I kept it. There is also the warning in everest.sh, but I'm
  not sure why it's checking for FSTAR_HOME in the first place.

  Most of this is untested... it does build, but I didn't run the workflows.

Note: For this PR to work with FSTAR_EXE alone (and no fstar.exe in the PATH), krml must also respect FSTAR_EXE (FStarLang/karamel#512). Or we would need a krml option to choose an fstar bypassing the detection.

[mtzguido]

Updated with the FSTAR_EXE ?= fstar.exe convention. We pass -fstar $(FSTAR_EXE) to krml when needed, and 3D also now has a --fstar option to override the F* used (otherwise we look in the PATH).

A run of the Github actions CI going on here https://github.com/mtzguido/everparse/actions/runs/12719269191.