sysutils/glow: Update to 1.2.1

Bugfix release, now filters markdowns asynchronously
so filtering doesn't block [1]

[1] https://github.com/charmbracelet/glow/releases/v1.2.1

PR: 251439
Submitted by: Lewis Cook <vulcan@wired.sh> (maintainer)
Approved by: arrowd, tcberner (mentors, implicit)