Make step deployment accessible as BITBUCKET_DEPLOYMENT variable in script

Environment variables available from pipeline steps are listed here:

https://confluence.atlassian.com/bitbucket/environment-variables-794502608.html

There doesn't seem to be a way to find out which deployment environment is set for this step at the moment (i.e., via fictional BITBUCKET_DEPLOYMENT variable). It is needed for deployment script to decide what to do; otherwise, steps can't be reused for different environments.

This enhancement is trivial to implement but will provide a great value, because it will dramatically facilitate the reuse of scripts, given that current abilities are limited to YAML anchors.