build: disable assistant_to_the_branch_manager on forks #2703

Job	Run time
list	21s
workflow (build-cli)	1m 36s
workflow (build-unit-tests)	2m 29s
workflow (devkit-core-tests)	1m 19s
workflow (modules-builder-tests)	2m 7s
workflow (devkit-core-tests-rerun)	1m 33s
	9m 25s

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