Add GitHub actions to the dependabot configuration. #122

Job	Run time
test / matrix-prep-os	4s
test / matrix-prep-bazelversion	9s
test / test (ubuntu-latest, 8.0.0, ., true)	51m 23s
test / test (ubuntu-latest, 7.3.1, ., true)	51m 43s
	1h 43m 19s

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