RFR: 2144: Bot image is only built for Linux x64

[Erik Duveblad]

Hi all,

please review this small patch that fixes the `bots` target in `build.gradle`. The problem is that the `bots` target in `build.gradle` tries to unpack a `.zip` file named after the local operating system and architecture, but we only ever build the bot jimage for Linux x64. The `bots` target should therefore only try to extract a `linux-x64` image.

### Testing
- [x] Tested locally on macOS aarch64

Thanks,
Erik

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Commit messages:
 - 2144

Changes: https://git.openjdk.org/skara/pull/1599/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1599&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2144
  Stats: 5 lines in 1 file changed: 0 ins; 3 del; 2 mod
  Patch: https://git.openjdk.org/skara/pull/1599.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1599/head:pull/1599

PR: https://git.openjdk.org/skara/pull/1599