checkInputs used to be added to nativeBuildInputs. Now we have
nativeCheckInputs to do that instead. Doing this treewide change allows
to keep hashes identical to before the introduction of
nativeCheckInputs.
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.
