Bot drops `build_num` context variable and overrides all build numbers

[isuruf]

In gdal-feedstock, due to a rattler-build bug prefix-dev/rattler-build#2497,
we had to override build number for all outputs. In order keep everything in sync, I used a context
variable. The bot undoes that and updates all the numbers instead of the context variable.

See conda-forge/gdal-feedstock@e8bff72