Fix gitian qt-win32.yml build

https://github.com/bitcoin/bitcoin/pull/3029
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.  Neither myself nor the reviewers caught this error.  I take responsibility for this mistake.

This commit is the minimal necessary to restore the qt win32 build, similar to how it worked previously.  I have personally tested rebuilding the entire stack from scratch and the result has a working deterministic bitcoin*.exe.

cfields has major cleanups for all gitian recipes coming soon, so please resist the urge to bikeshed this PR with more improvements as it is highly likely to be discarded in the coming rewrite.