nixos make-disk-image: fix permissions of /build

This was broken in 460c0d6 (PR #90431); now the nixos-unstable channel
should get unblocked.
vcunat modified this commit to use env-var instead of hardcoding /build

nixos/lib/make-disk-image.nix

@@ -181,6 +181,7 @@ let format' = format; in let
     export NIX_STATE_DIR=$TMPDIR/state
     nix-store --load-db < ${closureInfo}/registration
 
+    chmod 755 "$TMPDIR"
     echo "running nixos-install..."
     nixos-install --root $root --no-bootloader --no-root-passwd \
       --system ${config.system.build.toplevel} --channel ${channelSources} --substituters ""