Describe the bug
r.colors/v.colors module doesn't contains all colors options thumbnails.

To Reproduce
Steps to reproduce the behavior:

1. Launch r.colors/v.colors modules with --ui flag
2. Switch to Define tab (page) and visually check Name of color table ComboBox widget color options
3. Not all colors options have thumbnail (in the case when GRASS GIS was compiled with make -j option value > 1

Expected behavior
All color options should has thumbnails by default.

System description (please complete the following information):

• Operating System: all
• GRASS GIS version: all

Additional context
During compilation v.colors and r.colors module colors thumbnails are generated.

test@test~/src/grass$ grep -Rn "/utils/thumbnails.py"
raster/r.colors/Makefile:17:	-$(call run_grass, $(GRASS_HOME)/utils/thumbnails.py)
vector/v.colors/Makefile:15:	-$(call run_grass, $(GRASS_HOME)/utils/thumbnails.py)

Script for generation colors thumbnails https://github.com/OSGeo/grass/blob/main/utils/thumbnails.py used OS ENV vars which are mutually overwritten at compile time especially if make -j option value is > 1 and as a result, not all color options have a thumbnail.

Simple test:

1. Compilation with make -j4

GRASS nc_basic_spm_grass7/PERMANENT:~ > ls $GISBASE/docs/html/colortables | wc -l
38

Plus visually checking r.colors --ui/v.colors --ui modules Name of color table ComboBox widget color options thumbnails. Not all colors options have thumbnails.

2. Compilation with make -j1

GRASS nc_basic_spm_grass7/PERMANENT:~ > ls $GISBASE/docs/html/colortables | wc -l
56

Plus visually checking r.colors --ui/v.colors --ui modules Name of color table ComboBox widget color options thumbnails. All colors options have thumbnails.

Bug was originally reported here.