JENKINS-53232 Add possibility to take commit ID from environment variable in case the actual one is incorrect (for example due to "Merge before build"

[wimvr]

When using the "Merge before build", and a merge commit is performed, the plugin notifies Bitbucket with the commit ID of the merge commit, which is only known to Jenkins, not Bitbucket. This means the commit is not marked as successful build.
With this change you can override the commit ID with one stored in an environment variable.

JENKINS-53232

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• [*] Ensure you have provided tests - that demonstrates feature works or fixes the issue

• No tests have been added as plugin does not provide any tests yet.