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GH-39992: [CI][Docs][Java] ubuntu-docs uses Maven version in .env (#39993)

### Rationale for this change

GH-39696 updated Maven version but `ubuntu-docs` haven't used it yet.

### What changes are included in this PR?

Use `MAVEN` in `.env` in `ubuntu-docs`.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* Closes: #39992

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>