mk: Fix make install

Forgot to update the installation procedure with the knowledge that libcore is
only available as an rlib, not as a dylib.

Closes #14026
auto merge of #14033 : alexcrichton/rust/fix-mk-install, r=huonw

Forgot to update the installation procedure with the knowledge that libcore is
only available as an rlib, not as a dylib.

Closes #14026