Fix gitian qt-win32.yml build

The final revision of PR 3029 incorporated a review suggestion of replacing
i686-w64-mingw32 with $HOST. But we forgot that $HOST is not expanded when
within single quotes so it actually broke qt-win32.yml build.  This is the
minimal changes necessary to fix the qt-win32.yml build.
Merge pull request #3043 from wtogami/fixqtwin32

Fix gitian qt-win32.yml build