doc: use pure environment when building from git

* doc/contributing.texi (Building from Git): Add --pure to "guix environment"
  invocations

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/contributing.texi

@@ -57,14 +57,14 @@ all the dependencies and appropriate environment variables are set up to
 hack on Guix:
 
 @example
-guix environment guix
+guix environment guix --pure
 @end example
 
 @xref{Invoking guix environment}, for more information on that command.
 Extra dependencies can be added with @option{--ad-hoc}:
 
 @example
-guix environment guix --ad-hoc help2man git strace
+guix environment guix --pure --ad-hoc help2man git strace
 @end example
 
 Run @command{./bootstrap} to generate the build system infrastructure