Fix UUID extraction for root subvolume

[breztop]

ubuntu24.04
Adding boot menu entry for UEFI Firmware Settings ...
Detecting snapshots ...
UUID of the root subvolume is not available
If you think an error has occurred, please file a bug report at "https://github.com/Antynea/grub-btrfs"
done

[orpheus6678]

i faced the same issue just now on debian13.3, which comes with mawk by default. now, i commend the sincerity that went into mentioning gawk among the dependencies in the manual installation section of the dependencies, but people are likely to fall in this same pit like me :) there are multiple workarounds, like explicitly using gawk, or using posix compliant [[:space:]], or like the proposed change in this PR which reads much more nicely.

[Antynea]

Thanks for the report and the proposed fix.

A more complete fix covering the same issue has been submitted in #433, which addresses both UUID extraction lines and uses a cleaner approach. I'm closing this one in favor of that PR to consolidate the discussion there.

Feel free to continue following or commenting on #433.