sysutils/conky: update to 1.21.8

ChangeLog: https://github.com/brndnmtthws/conky/releases/tag/v1.21.8

Features

 * Implement -U. by @g0mb4 in #2049

Bug fixes

 * set display to null for race condition
 * src/fs.{cc,h}: Update fs_stats without error spam.
 * audacious.cc: print_audacious_filename(): fix always empty $audacious_filename
 * Fix tcp_ping for non-existent local network hosts
 * Fix if_match comparison for string operands containing operator symbols (#1988)
 * conky/pull/2064
 * fix: set initial state withdrawn wm hint (fixes #2046)

Miscellaneous

 * Bump version
 * build(deps): bump DeterminateSystems/nix-installer-action from 13 to 14 in the actions group
 * build(deps): bump DeterminateSystems/magic-nix-cache-action from 7 to 8 in the actions group
 * Add automatic labeling for issues
 * Revert "Add automatic labeling for issues"
 * build(deps): bump DeterminateSystems/nix-installer-action from 14 to 15 in the actions group