StashBuildEnvironmentContributor: Pass environment variables to pipelines

[proski]

*  StashBuildEnvironmentContributor: Pass environment variables to pipelines
   
   Add environment variables to every Run caused by StashCause even if it's
   not an instance of AbstractBuild.

Split from #69, as it's possible to set up a pipeline without relying on the environment variables provided by the plugin (although I would definitely recommend using sourceCommitHash to make sure the plugin tests what Bitbucket Server expects to be tested).

[proski]

I've added unit tests. The first commit adds unit tests for the existing code. The second commit changes the main code and updates the unit tests for the new behavior.

[jakub-bochenski]

I think we can just merge it independently since it doesn't require pipeline to work and improves the test coverage

[proski]

Fixed raw type warnings.