install-multi-user: chown bootstrap store contents

Before this change, the bootstrap store contents would be owned by the user who ran the script. This leads to inconsistencies in store path permissions, and can cause confusing issues for users (an example being zsh's compaudit being fired when attempting to use the completions from the bootstrap copy of Nix).

scripts/install-multi-user.sh

@@ -810,6 +810,24 @@ install_from_extracted_nix() {
         _sudo "to make the new store non-writable at $NIX_ROOT/store" \
               chmod -R ugo-w "$NIX_ROOT/store/"
 
+        # This is copied from create_directories, see it for why we do all this stuff just to find chown.
+        local get_chr_own="$(PATH="$(getconf PATH 2>/dev/null)" command -vp chown)"
+        if [[ -z "$get_chr_own" ]]; then
+            get_chr_own="$(command -v chown)"
+        fi
+
+        if [[ -z "$get_chr_own" ]]; then
+            reminder <<EOF
+I wanted to change ownership of new Nix store files,
+but I couldn't locate 'chown'. (You may need to fix your PATH.)
+To manually change file ownership, you can run:
+    sudo chown -R 'root:$NIX_BUILD_GROUP_NAME' '$NIX_ROOT'
+EOF
+        else
+            _sudo "to change ownership of Nix store files" \
+                  "$get_chr_own" -R "root:$NIX_BUILD_GROUP_NAME" "$NIX_ROOT" || true
+        fi
+
         if [ -d "$NIX_INSTALLED_NIX" ]; then
             echo "      Alright! We have our first nix at $NIX_INSTALLED_NIX"
         else