steam: add amd64 version and point url to fix version (as suggested by aristid).

Valve has a repository with fixed version packages, so use that instead of the
steam.deb I was using. They also have amd64 packages there.
steam: revert adding amd64 version.

It turns out that the .deb only contains the changelog and some other docs.
Revert back to using the i686 version, but keep the double url for the future.
steam: add flashplayer plugin support.

The videos work, but there is a problem with sound:
ALSA lib dlmisc.c:236:(snd1_dlobj_cache_get) Cannot open shared library
/nix/store/9z51hr9l19vdhgmqd60jwwrg6ny5md2d-alsa-plugins-1.0.26/lib/alsa-lib/libasound_module_pcm_pulse.so

It tries to open a 64bit plugin, which obviously fails. I think this depends on
/etc/asound.conf, but I'm not sure how to fix it.

Regardless of the problem, in one computer I tried the sound works but other
apps cannot output sound. In another, the sound doesn't work and other apps
continue working as normal.