Hadrian: Fix problem with unlit path in settings file

e529c65e introduced a problem in the logic for generating the
path to the unlit command in the settings file, and this patches
fixes it.

This fixes many tests, the simplest of which is:

> _build/stage1/bin/ghc testsuite/tests/parser/should_fail/T8430.lhs

which failed because of a wrong path for unlit, and now fails for the right
reason, with the error message expected for this test.

This addresses #16659.