[winswitch] [xpra] make the build reproducible

[Amit Biswas]

Whilst working on the Reproducible Builds effort [0], we noticed that
xpra could not be built reproducibly. This is due to the inclusion of
xpra/build_info.py file [1], which sets a variable BUILD_MACHINE to
the result of platform.uname(). The syscall is especially
unreproducible in containers, where it returns the instruction set of
the host kernel.

In addition to BUILD_MACHINE, other bits of build info like BUILD_CPU,
BUILD_TIME, BUILD_DATE in that file makes reproducibility challenging.
If you are open the the idea, I can provide a patch removing or
replacing them with alternatives like SOURCE_DATE_EPOCH [2]. Feel free
to respond with your thoughts on this.

 [0] https://reproducible-builds.org/
 [1] https://tests.reproducible-builds.org/debian/rb-pkg/unstable/i386/xpra.html
 [2] https://reproducible-builds.org/docs/source-date-epoch/

Thanks
Amit Biswas