[FLINK-40633][s3] Allow overriding the Maven executable in download-crt-jars.sh

[gaborgsomogyi]

What is the purpose of the change

The download-crt-jars.sh helper in flink-s3-fs-native hardcodes the mvn binary name, requiring plain Maven to be present on PATH. This fails in environments where only a Maven wrapper (or a differently named Maven executable) is available. This pull request adds an MVN environment variable override so the script can invoke any Maven executable, following the same MVN=${MVN:-mvn} convention already used in tools/releasing/*.sh.

Brief change log

• Introduced an MVN environment variable (defaulting to mvn) in download-crt-jars.sh
• Replaced the hardcoded mvn invocations with "${MVN}"
• Updated the script's PATH check and documentation comments accordingly

Verifying this change

Manually tested.
This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Claude code

[flinkbot]

CI report:

• 68235da Azure: FAILURE

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[Samrat002]

LGTM