Set preferLocalBuild for tests

tests/driver-linux.nix

@@ -8,13 +8,12 @@ let
   testDriver = with hostPkgs;
     stdenv.mkDerivation {
       name = "genode-test-driver";
+      preferLocalBuild = true;
 
       buildInputs = [ makeWrapper expect ];
 
       dontUnpack = true;
 
-      preferLocalBuild = true;
-
       installPhase = ''
         install -Dm555 ${./test-driver.exp} $out/bin/genode-test-driver
         wrapProgram $out/bin/genode-test-driver \
@@ -25,6 +24,7 @@ let
   runTests = driver:
     hostPkgs.stdenv.mkDerivation {
       name = "test-run-${driver.testName}";
+      preferLocalBuild = true;
 
       buildCommand = ''
         mkdir -p $out/nix-support

tests/driver-nova.nix

@@ -8,13 +8,12 @@ let
   testDriver = with hostPkgs;
     stdenv.mkDerivation {
       name = "nova-genode-test-driver";
+      preferLocalBuild = true;
 
       buildInputs = [ makeWrapper expect ];
 
       dontUnpack = true;
 
-      preferLocalBuild = true;
-
       installPhase = ''
         install -Dm555 ${./nova-test-driver.exp} $out/bin/genode-test-driver
         wrapProgram $out/bin/genode-test-driver \
@@ -25,6 +24,7 @@ let
   runTests = driver:
     hostPkgs.stdenv.mkDerivation {
       name = "test-run-${driver.testName}";
+      preferLocalBuild = true;
 
       buildCommand = ''
         mkdir -p $out/nix-support

tests/nova-iso.nix

@@ -62,6 +62,7 @@ let
 
 in hostPkgs.stdenv.mkDerivation {
   name = "nova-genode.iso";
+  preferLocalBuild = true;
   buildInputs = [ testPkgs.stdenv.cc hostPkgs.cdrkit ];
   dontUnpack = true;
   dontConfigure = true;