Instruct the user to follow redirects when installing Nix.

Nix installation now requires following redirects using `curl -L`. This
is currently represented on the [Nix download page][] but not in the
manual. This change updates the manual to reflect this.

Using `curl` without the `-L` flag results in an empty body, making
installation a no-op.

[Nix download page]: https://nixos.org/download.html

doc/manual/installation/env-variables.xml

@@ -39,7 +39,7 @@ bundle.</para>
   <step><para>Set the environment variable and install Nix</para>
     <screen>
 $ export NIX_SSL_CERT_FILE=/etc/ssl/my-certificate-bundle.crt
-$ sh &lt;(curl https://nixos.org/nix/install)
+$ sh &lt;(curl -L https://nixos.org/nix/install)
 </screen></step>
 
   <step><para>In the shell profile and rc files (for example,

doc/manual/installation/installing-binary.xml

@@ -12,7 +12,7 @@
 </para>
 
 <screen>
-  $ sh &lt;(curl https://nixos.org/nix/install)
+  $ sh &lt;(curl -L https://nixos.org/nix/install)
 </screen>
 
 <para>
@@ -39,7 +39,7 @@
     To explicitly select a single-user installation on your system:
 
     <screen>
-  sh &lt;(curl https://nixos.org/nix/install) --no-daemon
+  sh &lt;(curl -L https://nixos.org/nix/install) --no-daemon
 </screen>
   </para>
 

doc/manual/introduction/quick-start.xml

@@ -15,7 +15,7 @@ to subsequent chapters.</para>
 <step><para>Install single-user Nix by running the following:
 
 <screen>
-$ bash &lt;(curl https://nixos.org/nix/install)
+$ bash &lt;(curl -L https://nixos.org/nix/install)
 </screen>
 
 This will install Nix in <filename>/nix</filename>. The install script