Loader improvements #763

Job	Run time
Build and package cores	7m 25s
Split off ${{ matrix.artifact }}	0s
verify-core	3s
Test ${{ matrix.subarch }}:${{ matrix.board }}	-1s
Collect logs	-1s
Publish jsons	-1s
cleanup-artifacts	4s
Publish core	-1s
	7m 28s

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