operations: Always set GL_PROTOCOL in hooks

When updating references via updateReferenceWithHooks, then we need to assemble the envvars to post to the service. But one envvar actually got lost in the conversion from Ruby to Go, which is the GL_PROTOCOL variable. Not setting this variable may cause GitLab to misinterpret a change's source and thus fail to correctly apply access control rules.

Add the missing variable. As the function will only ever be called from other RPCs in the OperationsService, its safe to assume that it'll always we "web".

Fixes #3271 (closed)