fix license ci by using merge base #22127

Job	Run time
licensing	5m 28s
pyflakes	3m 38s
cancel	7s
Check if image exists in registry	8s
linux (ubuntu-20.04, debug)	43m 41s
linux-fpga (ubuntu-20.04, debug)	42m 11s
linux (ubuntu-20.04, release)	7m 22s
linux (ubuntu-20.04, codecov)	10m 24s
misspell	26s
Build SLES image	0s
Build image	0s
sles	38m 32s
cppcheck	1h 0m 49s
format	5m 24s
tidy	7m 46s
	3h 45m 56s

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