i2c_launcher: Dehardcode battery index #29637

Job	Run time
build (tests)	5m 45s
build (tests_coverage)	7m 25s
build (check_format)	1m 12s
build (px4_fmu-v2_default stack_check)	3m 14s
build (check_newlines)	1m 34s
build (NO_NINJA_BUILD=1 px4_sitl_default)	3m 26s
build (validate_module_configs)	58s
build (NO_NINJA_BUILD=1 px4_fmu-v5_default)	4m 12s
build (module_documentation)	1m 4s
build (shellcheck_all)	1m 46s
build (px4_sitl_allyes)	4m 30s
	35m 6s

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