nixos/manual: use system nixpkgs to build pxe image

The command in example is expected to be run from nixpkgs checkout, but
there's no explanation of this. Let's just use system nixpkgs: most
users will have it just working and those who use git checkouts will
figure it out.

Changed files
+4 -4
nixos
doc
manual
from_md
installation
+2 -2
nixos/doc/manual/from_md/installation/installing-pxe.section.xml
···
<para>
These instructions assume that you have an existing PXE or iPXE
infrastructure and simply want to add the NixOS installer as another
-
option. To build the necessary files from a recent version of
+
option. To build the necessary files from your current version of
nixpkgs, you can run:
</para>
<programlisting>
-
nix-build -A netboot.x86_64-linux nixos/release.nix
+
nix-build -A netboot.x86_64-linux '&lt;nixpkgs/nixos/release.nix&gt;'
</programlisting>
<para>
This will create a <literal>result</literal> directory containing: *
+2 -2
nixos/doc/manual/installation/installing-pxe.section.md
···
These instructions assume that you have an existing PXE or iPXE
infrastructure and simply want to add the NixOS installer as another
-
option. To build the necessary files from a recent version of nixpkgs,
+
option. To build the necessary files from your current version of nixpkgs,
you can run:
```ShellSession
-
nix-build -A netboot.x86_64-linux nixos/release.nix
+
nix-build -A netboot.x86_64-linux '<nixpkgs/nixos/release.nix>'
```
This will create a `result` directory containing: \* `bzImage` -- the