arrow

GH-15282: [CI][C++] add CLANG_TOOLS variable in .travis.yaml (#32972)

# Which issue does this PR close?

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If this is not a [minor PR](https://github.com/apache/arrow/blob/master/CONTRIBUTING.md#Minor-Fixes). Could you open an issue for this pull request on GitHub? https://github.com/apache/arrow/issues/new/choose

Opening GitHub issues ahead of time contributes to the [Openness](http://theapacheway.com/open/#:~:text=Openness%20allows%20new%20users%20the,must%20happen%20in%20the%20open.) of the Apache Arrow project.

Then could you also rename the pull request title in the following format?

    GH-${GITHUB_ISSUE_ID}: [${COMPONENT}] ${SUMMARY}

or

    MINOR: [${COMPONENT}] ${SUMMARY}

In the case of old issues on JIRA the title also supports:

    ARROW-${JIRA_ISSUE_ID}: [${COMPONENT}] ${SUMMARY}
    PARQUET-${JIRA_ISSUE_ID}: [${COMPONENT}] ${SUMMARY}

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Closes #15282 

# Rationale for this change

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This change fixes a version error for downloading clang in apt. Recent scripts require two variables `LLVM` and `CLANG_TOOLS` are defined in [`.env`](https://github.com/apache/arrow/blob/master/.env#L56-L57).

* Closes: #15282

Authored-by: Kazuaki Ishizaki <ishizaki@jp.ibm.com>
Signed-off-by: Jacob Wujciak-Jens <jacob@wujciak.de>