Backport of https://github.com/NixOS/nix/pull/12275
nix-env can read priorities from a derivations meta attributes, but this
only works when installing a nix expression.
nix-env can also install bare store paths, however meta attributes are
not readable in that case. This means that a store path can not be
installed with a specific priority.
Some cases where it is advantageous to install a store path: a remote
host following a `nix copy`, or any time you want to save some
evaluation time and happen to already know the store path.
This PR addresses this shortcoming by adding a --priority flag to
nix-env --install.
Change-Id: Ibd9365d0058820a9c2aeecc909b81a0410d5764b
Andrew Hamon
2bdac6b7ec