GNUmakefile: avoid bootstrap issue with mkdir

[oech3]

Support the system mkdir is broken via INSTALL="cargo run ...".

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/io-errors (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/tail/tail-n0f is now passing!

[cakebaker]

Hm, somehow I don't understand the reason for the changes of this PR. Why is using install "better" than using mkdir?

[oech3]

This makes easier to install uutils on the system missing system coreutils by some reason. We only need to build uu_install at 1st.

[cakebaker]

Thanks!

[oech3]

Can we just remove sort too?

[cakebaker]

Probably yes.

[oech3]

It might make's built-in...