nix-env: add a --priority flag to --install

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.

doc/manual/source/command-ref/nix-env/install.md

@@ -11,6 +11,7 @@
   [`--from-profile` *path*]
   [`--preserve-installed` | `-P`]
   [`--remove-all` | `-r`]
+  [`--priority` *priority*]
 
 # Description
 
@@ -61,6 +62,10 @@ The arguments *args* map to store paths in a number of possible ways:
   The derivations returned by those function calls are installed.
   This allows derivations to be specified in an unambiguous way, which is necessary if there are multiple derivations with the same name.
 
+- If `--priority` *priority* is given, the priority of the derivations being installed is set to *priority*.
+  This can be used to override the priority of the derivations being installed.
+  This is useful if *args* are [store paths], which don't have any priority information.
+
 - If *args* are [store derivations](@docroot@/glossary.md#gloss-store-derivation), then these are [realised], and the resulting output paths are installed.
 
 - If *args* are [store paths] that are not store derivations, then these are [realised] and installed.
@@ -235,4 +240,3 @@ channel:
 ```console
 $ nix-env --file https://github.com/NixOS/nixpkgs/archive/nixos-14.12.tar.gz --install --attr firefox
 ```
-