This is a follow-up for #460057 to completely remove all mentions of nixos.org/channels/.
The following command was used to generate this change:
```
find . -type f -exec sed -i 's|nixos.org/channels/|channels.nixos.org/|g' {} +
```
Should now work equally well with bash (as on NixOS) and dash (on
various other distributions).
The only truly required change was on line 10: "function" is a
superfluous keyword that's not present in posix sh.
Change-Id: If917499b2e24c0d039e8c61208227b90c9fc5c0d
