stdenv.targetPlatform really shouldn't be used by software that
doesn't generate or manipulate binaries. I reviewed all uses of
targetPlatform outside of pkgs/development/compilers and pkgs/stdenv
and replaced those which weren't involved in something which fits
these criteria.
The xdg-open utility is only ever a runtime dependency and its
dependents only expect that it accept a URI as a command line
argument and do something with it that the user would expect.
For such as a trivial relationship it should be possible for
users to override xdg-open with something else in their PATH.
