cargo: Add system mode for installing to sysroot

[brson]

There's some code for this in cargo now, but it doesn't quite do the right thing. See #1760.

[brson]

Also worth considering the difference between sysroot and the install prefix - they are not the same. For instance, I never install cargo, always running it out of the build directory, in which case the build directory is sysroot.

[brson]

I think this is resolved.