Fix "permission denied" issue with gradlew and MavenW

[eranturgeman]

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[attiasas]

Nice Job!

Consider:

🟡 [Correctness]
mvn.go uses exec.Command("./mvnw", goals...) directly — the same pattern that caused the gradlew bug.

Apply the same sh workaround to RunMvnCmd for the wrapper path (or extract a shared helper).

[github-actions]

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