Add `--override-build-num` flag #491

Job	Run time
Test on anaconda_recipes_01	3m 31s
Test on bioconda_recipes_01	3m 32s
Test on bioconda_recipes_02	3m 41s
Test on bioconda_recipes_03	2m 59s
Test on bioconda_recipes_04	2m 55s
Test on conda_forge_recipes_01	3m 53s
Integration Testing Final Report	1m 11s
	21m 42s

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