prefer-fetch-remote: an overlay to fetch on remote builders

This is useful when running tools like NixOps or nix-review
on workstations where the upload to the builder is significantly
slower then downloading the source on the builder itself.

pkgs/build-support/fetchsvn/default.nix

@@ -1,6 +1,7 @@
 {stdenvNoCC, subversion, glibcLocales, sshSupport ? false, openssh ? null}:
-{url, rev ? "HEAD", md5 ? "", sha256 ? "",
- ignoreExternals ? false, ignoreKeywords ? false, name ? null}:
+{url, rev ? "HEAD", md5 ? "", sha256 ? ""
+, ignoreExternals ? false, ignoreKeywords ? false, name ? null
+, preferLocalBuild ? true }:
 
 let
   repoName = with stdenvNoCC.lib;
@@ -40,5 +41,5 @@ stdenvNoCC.mkDerivation {
   inherit url rev sshSupport openssh ignoreExternals ignoreKeywords;
 
   impureEnvVars = stdenvNoCC.lib.fetchers.proxyImpureEnvVars;
-  preferLocalBuild = true;
+  inherit preferLocalBuild;
 }