KAFKA-20379: ducker-ak has no option to skip the gradlew systemTestLibs build step

[see-quick]

This PR adds an option to skip the gradle build while running system tests.

Why

Add --skip-build support to ducker-ak test. Currently when someone wants to skip the build it (e.g., it is already builded in the CI) it cannot be skip because:

kafka/tests/docker/ducker-ak

Line 607 in acd37fc

( (test -f ./gradlew || gradle) && ./gradlew systemTestLibs ) || die "ducker_test: Failed to build system test libraries, please check the error log."

is always called in the ducker_test method in the script.

Important

Additionally, run_tests.sh has a redundant REBUILD block (i.e.,:

kafka/tests/docker/run_tests.sh

Lines 43 to 48 in acd37fc

	if [ "$REBUILD" == "t" ]; then
	./gradlew clean systemTestLibs
	if [ "$KAFKA_MODE" == "native" ]; then
	./gradlew clean releaseTarGz
	fi
	fi

and think is a dead code because ducker-ak test always rebuilds anyway (no matter what you set) here:

kafka/tests/docker/run_tests.sh

Line 56 in acd37fc

${SCRIPT_DIR}/ducker-ak test ${TC_PATHS} ${_DUCKTAPE_OPTIONS} || die "ducker-ak test failed"

Tested

1. SKIP_BUILD=true ./tests/docker/ducker-ak test ./tests/kafkatest/tests/core/produce_bench_test.py => build is skipped
2. ./tests/docker/ducker-ak test ./tests/kafkatest/tests/core/produce_bench_test.py => build is NOT skipped
3. ./tests/docker/ducker-ak test --skip-build ./tests/kafkatest/tests/core/produce_bench_test.py => build is skipped
4. SKIP_BUILD=true ./tests/docker/run_tests.sh => build is skipped
5. ./tests/docker/run_tests.sh => build is NOT skipped
6. ... (more)

Again as previous --memory option, --skip-build option has same priority (i.e., CLI > ENV > default option).

[see-quick]

Double-check please this one (so I might miss something ...), but I think it is not needed as I think described this in the PR description.