Fix digi legs sprites getting stuck (#87254)

## About The Pull Request

Fixes #86756
Fixes #67174 (isn't this already fixed? Oh well now it's really fixed
because I tested it)

Basically changed the random calls to `update_body_parts` with a bespoke
proc which better explains what they are there to do

Adds one missing call to head items (for `HIDESNOUT`)

## Changelog

🆑 Melbert
fix: Fixed digitigrade pants sprite not updating in accordance to some
leg updates
/🆑
# Conflicts:
#	code/modules/mob/living/carbon/human/human_update_icons.dm
#	code/modules/surgery/bodyparts/species_parts/lizard_bodyparts.dm

code/modules/unit_tests/_unit_tests.dm

@@ -240,6 +240,7 @@
 #include "say.dm"
 #include "screenshot_antag_icons.dm"
 #include "screenshot_basic.dm"
+#include "screenshot_digi.dm"
 #include "screenshot_dynamic_human_icons.dm"
 #include "screenshot_high_luminosity_eyes.dm"
 #include "screenshot_humanoids.dm"