Add an option ‘use-binary-caches’

This allows disabling the use of binary caches, e.g.

  $ nix-build ... --option use-binary-caches false

Note that

  $ nix-build ... --option binary-caches ''

does not disable all binary caches, since the caches defined by
channels will still be used.

doc/manual/conf-file.xml

@@ -318,6 +318,16 @@ build-use-chroot = /dev /proc /bin</programlisting>
   </varlistentry>
 
 
+  <varlistentry><term><literal>use-binary-caches</literal></term>
+
+    <listitem><para>If set to <literal>true</literal> (the default),
+    Nix will check the binary caches specified by
+    <option>binary-caches</option> and related options to obtain
+    binary substitutes.</para></listitem>
+
+  </varlistentry>
+
+
   <varlistentry><term><literal>binary-caches</literal></term>
 
     <listitem><para>A list of URLs of binary caches, separated by

scripts/download-from-binary-cache.pl.in

@@ -177,6 +177,10 @@ sub getAvailableCaches {
     return if $gotCaches;
     $gotCaches = 1;
 
+    return if
+        ($Nix::Config::config{"use-binary-caches"} // "true") eq "false" ||
+        ($Nix::Config::config{"untrusted-use-binary-caches"} // "true") eq "false";
+
     sub strToList {
         my ($s) = @_;
         return map { s/\/+$//; $_ } split(/ /, $s);