[iam] Fix incorrect username used on bitbucket

Created by: kopancek

Based on original suggestion from @unknwon #40241

fixes: https://github.com/sourcegraph/customer/issues/1161

Test plan

Tested with unit tests.

Tested locally against the bitbucket server instance https://bitbucket.sgdev.org/ and it worked with the default configuration there.

Tested locally against a bitbucket server instance with the same config as customer. Mentioned in this comment: https://github.com/sourcegraph/customer/issues/1161#issuecomment-1216217367