Downgrades native maven plugin. See #336 for context.

Adds additional switch case to build.sh to accommodate native image testing.
Source-Link: googleapis/synthtool@d45942b
Post-Processor: gcr.io/repo-automation-bots/owlbot-java:latest@sha256:d4b2141d65566523dfd523f63c6e6899ab1281463bce182a9f600e74b0511875

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply..com>

Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply..com>

🤖 I have created a release \*beep\* \*boop\*
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## [1.2.0](https://www..com/googleapis/java-shared-config/compare/v1.1.0...v1.2.0) (2021-10-21)


### Features

* Introduce Native Image testing build script changes ([#1240](https://www..com/googleapis/java-shared-config/issues/1240)) ([#334](https://www..com/googleapis/java-shared-config/issues/334)) ([4643cf1](https://www..com/googleapis/java-shared-config/commit/4643cf15be0b37fa8fa905d544d438cda7ef2ecd))


### Bug Fixes

* **java:** downgrade native maven plugin version ([#335](https://www..com/googleapis/java-shared-config/issues/335)) ([5834284](https://www..com/googleapis/java-shared-config/commit/5834284176fb34713d10082ce04f3b6abba85ad8))
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