* Source vendoring is passed as a standalone attribute to
`mkCargoDerivation`, meaning it does not automagically get spliced
with the correct local/cross system inputs (i.e. what would happen if
it was a `depsBuildBuild` entry)
* To fix this we need to make sure that `vendorCargoDeps` and all of its
transitive dependencies always use `runCommand` (and friends) from
their `pkgsBuildBuild` equivalent. This should always be safe to do
(even for cross-builds) since this amounts to building up a bunch of
sources which will be read by the build system
* Unfortunately I had to manually specify `pkgsBuildBuild.whatever` in
multiple places as I could not get things to work by messing with the
`callPackage` definition. Perhaps we should be using
`makeScopeWithSplicing'` instead of `makeScope` when constructing the
library, but I couldn't get it working (and I couldn't find any decent
docs on how to use it online) so this will make do for the time being.
