by the build farm. See e.g.,
http://catamaran.labs.cs.uu.nl/dist/nixpkgs-0.8/nixpkgs-0.7pre2302/;
the user can click on packages, and they will be installed (assuming
the `application/nix-package' MIME type has been associated with
`nix-install-package').
Nix expressions are no longer involved: a "package" is just a
pointer to a manifest, and the top-level store derivation to be
added to the user environment. This makes these packages
independent from Nix expression evolution.
Note that we install the store derivation ($drvPath), not the
resulting output path ($outPath). This is equivalent, except that
installing the derivation maintains the back-link from the output
path to the derivation that built it. This is useful for
maintenance.
* Automatically re-exec in an xterm so that the user sees something
when `nix-install-package' is run from a browser.
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