doc: Use Awk in one of the examples.

The output of the listing of available packages now contains a mix of tab and
space characters, so that its output is more pleasing to read.  This preserves
most simple uses of 'cut', but the added extra space padding can cause a
change in behavior in some scenarios.

* doc/guix.texi (Invoking guix build): Replace 'cut' by 'awk' in one of the
examples.

doc/guix.texi

@@ -10227,7 +10227,7 @@ Similarly, the following command builds all the available packages:
 
 @example
 guix build --quiet --keep-going \
-  $(guix package -A | cut -f1,2 --output-delimiter=@@)
+  $(guix package -A | awk '@{ print $1 "@@" $2 @}')
 @end example
 
 @var{package-or-derivation} may be either the name of a package found in