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
Add compiler flag to force color output

This adds the flag --color. Why this is beneficial is explained in
issue #12881.

Please note that this commit doesn't include any regression tests. I
thought about how I'd write a test for this and it doesn't seem to be
worth the effort to me for a UI change like this.

Fixes #12881.