Fix script sha256sum verification on macOS

[emilime93]

I found a vulnerability/bug in the mvnw script on macOS where malformed SHA-256 checksums in the distributionSha256Sum property may pass validation despite being invalid. This only impacts cases where the checksum is incorrectly formatted, not when it's correctly formatted but incorrect.

This occurs because the script invokes sha256sum without the --strict flag, and macOS's Darwin implementation does not enforce strict format checking by default, unlike GNU coreutils. When a malformed checksum is provided (e.g., an extra hex character), sha256sum emits a warning to stderr but returns a success exit code, which the script interprets as valid. Since stderr is redirected to /dev/null, the warning is silenced. The issue affects both mvnw and only-mvnw scripts. The fix adds the --strict flag to ensure improperly formatted checksum lines cause validation to fail, aligning macOS behavior with Linux.

Unfortunately I did not find an easy way to make a test for this.

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