nixos-install: error out if $mountPoint has bad permissions

The nix store more-or-less requires o+rx on all parent directories.
This is primarily because nix runs builders in a uid/gid mapped
user-namespace, and those builders have to be able to operate on the nix
store.

This check is especially helpful because nix does not produce a helpful
error on its own (rather, creating directories and such works, it's not
until 'mount --bind' that it gets an EACCES).

Helps users who run into this opaque error, such as in #67465.
Possibly fixes that issue if bad permissions were the only cause.

nixos/modules/installer/tools/nixos-install.sh

@@ -71,6 +71,17 @@ if ! test -e "$mountPoint"; then
     exit 1
 fi
 
+# Verify permissions are okay-enough
+checkPath="$(realpath "$mountPoint")"
+while [[ "$checkPath" != "/" ]]; do
+    mode="$(stat -c '%a' "$checkPath")"
+    if [[ "${mode: -1}" -lt "5" ]]; then
+        echo "path $checkPath should have permissions 755, but had permissions $mode. Consider running 'chmod o+rx $checkPath'."
+        exit 1
+    fi
+    checkPath="$(dirname "$checkPath")"
+done
+
 # Get the path of the NixOS configuration file.
 if [[ -z $NIXOS_CONFIG ]]; then
     NIXOS_CONFIG=$mountPoint/etc/nixos/configuration.nix