rustbuild lacks make install

[Aatch]

I decided to switch to using rustbuild recently and discovered that there is no make install for rustbuild. This is somewhat annoying as it's not very clear how exactly to do a full install of the just-built compiler given what's there.

[alexcrichton]

Oh dear, this is a very good point! I ended up doing 90% of the work without going all the way there! So it turns out that make install is actually a very thin wrapper around make prepare which in rustbuild is just --step dist. This should definitely be implemented!

The step for implementing this are likely along the lines of:

• Add an install target to this array with the one argument of a `stage.
• Modify this function to say that the dependency of the install target is the dist target
• Add a case to this loop to dispatch the to the right step
• Add an install function to dist.rs most likely to install local artifacts.
• In the install function, extract the tarball created by the dist step, run the install.sh script for the std pkg, the rustc pkg, and optionally the docs pkg

This shouldn't be too hard but I'm not going to tag this with E-easy as it's somewhat difficult. Especially around handling sudo vs not-sudo I think could get a little tricky, but the gist is that we should probably just refuse a bootstrap build or rustbuild build if the $USER is root, having make install delegate to the right user to prepare the installation. Then the make install target could just make sure to install to the right location regardless of the user.

[alexcrichton]

As pointed out by @jirutka we should be careful to respect GNU standards here for environment variables and such when implementing this.

[alexcrichton]

Fixed in #35641