steam: Do $PATH lookup in steam.desktop instead of hardcoding derivation
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/<derivation hash>-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this means such copies will point at wrong/outdated derivations once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).

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