depends: Use more legible qmake commands in qt package

Summary:

depends: Do not set build_subdir for qt package

This change makes the next commit possible without exporting the PATH
variable.

depends: Use more legible qmake commands in qt package
This change gets rid of multiple ../ that makes reasoning about the
script and its maintaining much easier.

Co-authored-by: fanquake <fanquake@gmail.com>

See D830 for the differences in the source material.

This is a backport of core#20673

Depends on D13664

Test Plan: gitian builds

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13665