doc: change git:// links to https:// (#44395)

The server is not verified over the git:// transfer protocol. If you
clone a repository over git://, you should check if the latest commit's
hash is correct.

On the other hand, https:// will always verify the server automatically,
using certificate authorities.

README.md

@@ -8,7 +8,7 @@ build daemon as so-called channels. To get channel information via git, add
 [nixpkgs-channels](https://github.com/NixOS/nixpkgs-channels.git) as a remote:
 
 ```
-% git remote add channels git://github.com/NixOS/nixpkgs-channels.git
+% git remote add channels https://github.com/NixOS/nixpkgs-channels.git
 ```
 
 For stability and maximum binary package support, it is recommended to maintain

doc/quick-start.xml

@@ -9,7 +9,7 @@
     <para>
      Checkout the Nixpkgs source tree:
 <screen>
-$ git clone git://github.com/NixOS/nixpkgs.git 
+$ git clone https://github.com/NixOS/nixpkgs
 $ cd nixpkgs</screen>
     </para>
    </listitem>

nixos/doc/manual/configuration/adding-custom-packages.xml

@@ -14,7 +14,7 @@
 xlink:href="http://nixos.org/nixpkgs/manual">Nixpkgs
   manual</link>. In short, you clone Nixpkgs:
 <screen>
-$ git clone git://github.com/NixOS/nixpkgs.git
+$ git clone https://github.com/NixOS/nixpkgs
 $ cd nixpkgs
 </screen>
   Then you write and test the package as described in the Nixpkgs manual.

nixos/doc/manual/development/sources.xml

@@ -11,9 +11,9 @@
   modify NixOS, however, you should check out the latest sources from Git. This
   is as follows:
 <screen>
-$ git clone git://github.com/NixOS/nixpkgs.git
+$ git clone https://github.com/NixOS/nixpkgs
 $ cd nixpkgs
-$ git remote add channels git://github.com/NixOS/nixpkgs-channels.git
+$ git remote add channels https://github.com/NixOS/nixpkgs-channels
 $ git remote update channels
 </screen>
   This will check out the latest Nixpkgs sources to