Bug#916301: steam: cannot be built with debuild -ai386 on an amd64 system

[Simon McVittie]

Package: steam
Version: 1.0.0.56-1
Severity: minor
Tags: patch

If you have an amd64 system (as I would expect essentially all Steam
users do), the Debian steam package can't be built with debuild -ai386
due to two minor issues:

* the Build-Depends on python3 wants a host-architecture (i386) version
  of python3, but should use python3:any to accept a build-architecture
  (amd64) version, since we are not compiling Python extensions

* d/rules mixes up DEB_BUILD_ARCH with DEB_HOST_ARCH

Patch attached.

    smcv