linuxManualConfig: don't build inside source tree

We can avoid the need to explicitly exclude it later if we just put it
somewhere else to begin with.

pkgs/os-specific/linux/kernel/manual-config.nix

@@ -156,8 +156,7 @@ let
       configurePhase = ''
         runHook preConfigure
 
-        mkdir build
-        export buildRoot="$(pwd)/build"
+        export buildRoot=$(mktemp -d)
 
         echo "manual-config configurePhase buildRoot=$buildRoot pwd=$PWD"
 
@@ -275,8 +274,8 @@ let
         mkdir -p $dev/lib/modules/${modDirVersion}/{build,source}
 
         # To save space, exclude a bunch of unneeded stuff when copying.
-        (cd .. && rsync --archive --prune-empty-dirs \
-            --exclude='/build/' \
+        (cd "$NIX_BUILD_TOP" && cd "$sourceRoot" &&
+         rsync --archive --prune-empty-dirs \
             * $dev/lib/modules/${modDirVersion}/source/)
 
         cd $dev/lib/modules/${modDirVersion}/source