mk: Simplify how prepare.mk, install.mk, and dist.mk deal with stages

The only stage that can be installed from is 2 everywhere but windows,
3 on windows.

Closes #12799
auto merge of #13024 : brson/rust/issue-12799, r=thestinger

The only stage that can be installed from is 2 everywhere but windows,
3 on windows.

Lightly tested. Not actually tested on Windows, but I did confirm that a *similar* change fixed the problem on Windows.

Closes #12799