fetchsvn: Fix for cross

Just use `nativeBuildInputs` at build time.

pkgs/build-support/fetchsvn/builder.sh

@@ -2,10 +2,6 @@ source $stdenv/setup
 
 header "exporting $url (r$rev) into $out"
 
-if test "$sshSupport"; then
-    export SVN_SSH="$openssh/bin/ssh"
-fi
-
 if test -n "$http_proxy"; then
     # Configure proxy
     mkdir .subversion

pkgs/build-support/fetchsvn/default.nix

@@ -1,7 +1,13 @@
-{stdenvNoCC, subversion, glibcLocales, sshSupport ? true, openssh ? null}:
+{ stdenvNoCC, buildPackages
+, subversion, glibcLocales, sshSupport ? true, openssh ? null
+}:
+
 { url, rev ? "HEAD", md5 ? "", sha256 ? ""
 , ignoreExternals ? false, ignoreKeywords ? false, name ? null
-, preferLocalBuild ? true }:
+, preferLocalBuild ? true
+}:
+
+assert sshSupport -> openssh != null;
 
 let
   repoName = with stdenvNoCC.lib;
@@ -32,13 +38,16 @@ else
 stdenvNoCC.mkDerivation {
   name = name_;
   builder = ./builder.sh;
-  nativeBuildInputs = [ subversion glibcLocales ];
+  nativeBuildInputs = [ subversion glibcLocales ]
+    ++ stdenvNoCC.lib.optional sshSupport openssh;
+
+  SVN_SSH = if sshSupport then "${buildPackages.openssh}/bin/ssh" else null;
 
   outputHashAlgo = "sha256";
   outputHashMode = "recursive";
   outputHash = sha256;
 
-  inherit url rev sshSupport openssh ignoreExternals ignoreKeywords;
+  inherit url rev ignoreExternals ignoreKeywords;
 
   impureEnvVars = stdenvNoCC.lib.fetchers.proxyImpureEnvVars;
   inherit preferLocalBuild;