KAFKA-20531: ducker-ak: Add --clean-build option to replace REBUILD env var

[ryan-cho-i]

Adds a --clean-build / -c flag to both ducker-ak up and
ducker-ak test, providing a discoverable CLI replacement for the
REBUILD=t environment variable in tests/docker/run_tests.sh.

What changed

tests/docker/ducker-ak

• prepare_native_dir() now accepts an optional force_rebuild
  argument. When set, the Kafka release tarball is rebuilt even
  if a cached one already exists, addressing the stale-tarball
  problem when source code has changed since the last build.
• ducker_up parses -c|--clean-build and forwards it to
  prepare_native_dir. Has no effect in JVM mode.
• ducker_test parses -c|--clean-build and runs
  ./gradlew clean systemTestLibs instead of the incremental build.
• Help text updated for both up and test.

tests/docker/run_tests.sh

• Existing REBUILD=t behavior is preserved for backward
  compatibility, but the path now emits a deprecation warning
  pointing to the new flag.
• Added a TODO marker indicating the env var should be removed
  in 5.0.

Why split across up and test?

The two commands handle different caches:

• ducker-ak up's prepare_native_dir produces the Kafka native
  tarball that gets baked into the Docker image. Stale here means
  testing against an outdated binary.
• ducker-ak test builds systemTestLibs, which is consumed at
  ducktape execution time inside the already-running cluster.

Adding the flag to both keeps the responsibility split intact and
matches the description in the JIRA issue ("forces native tarball
rebuild in prepare_native_dir").

Testing

• bash -n passes on both modified files.
• ./tests/docker/ducker-ak --help correctly shows the new flag
  under both up and test sections.
• Verified deprecation warning fires when REBUILD=t is set on
  run_tests.sh.

Follow-up

A separate ticket can track the removal of the REBUILD=t code
path in 5.0.