Mercurial/Rust: fallbacking to install

Actually, `pip --target` has the effect of not providing
the main executable, whereas `--install-lib` makes a better
distinction. It still has to be supplemented with
`--prefix=/usr/local`, otherwise the resulting `hg` would
still be in `/usr/bin`, which is not expected by users of
this image (and a discrepancy with builds without Rust,
that could work with a simple `pip install Mercurial`).

I'm getty too old for this…
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