Note: I received this patch via E-Mail. As I'm not using any of the
"fancy" DEs I can't really judge if this is how and what we want to do
here.

steam: Do $PATH lookup in steam.desktop instead of hardcoding derivation

The desktop application and the absoloute path work fine.
But consider desktop environments such as KDE where, in the application
menu, one can right click entries and pin them to widgets/panels, add
them to the desktop, etc.

Doing so effectively means copying
/run/current-system/sw/share/applications/steam.desktop to
~/.local/share/plasma_icons/ or ~/Desktop/, i.e. managed stated gets
duplicated outside the nix scope.

The problem here is that steam.desktop hardcodes

Exec=/nix/store/-steam/bin/steam %U

this means such copies will point at wrong/outdated derivations once
the steam package changes, i.e. widgets/panels/desktop icons will no
longer work and must be recreated.

Therefore replace the absoloute path with a $PATH lookup to allow "safe"
copying; this isn't optimal but other applications such Firefox and
Thunderbrid currently behave the same way ($PATH lookup in their
.desktop file).

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS - nix-build -A steam