install-multi-user: ignore profile_target backups that have no change

If there was a prior nix installation that created this backup file and
then you tried to install it again, it would stop to tell you there is
this file. But if the file and its backup are identical in content,
there is no harm in continuing and in a later step overwriting the
existing backup file with the identical one. This is just a convenience
feature.

scripts/install-multi-user.sh

@@ -445,6 +445,14 @@ EOF
         #       a row for different files.
         if [ -e "$profile_target$PROFILE_BACKUP_SUFFIX" ]; then
             # this backup process first released in Nix 2.1
+
+            if diff -q "$profile_target$PROFILE_BACKUP_SUFFIX" "$profile_target" > /dev/null; then
+                # a backup file for the rc-file exist, but they are identical,
+                # so we can safely ignore it and overwrite it with the same
+                # content later
+                continue
+            fi
+
             failure <<EOF
 I back up shell profile/rc scripts before I add Nix to them.
 I need to back up $profile_target to $profile_target$PROFILE_BACKUP_SUFFIX,