Slug not available on Jenkins

[vio]

The slug value is missing on Jenkins https://github.com/semantic-release/env-ci/blob/master/services/jenkins.js, but the documentation is showing that it is provided:

Jenkins does not provide a slug env variable, but a possible workaround for git projects is to extract it from GIT_URL. Happy to open a PR if it is helpful to have it on env-ci.

Environment:

• CI: Jenkins

Documentation:

• Jenkins env variables: https://www.jenkins.io/doc/book/pipeline/jenkinsfile/#using-environment-variables
• Jenkins git-plugin env variables: https://plugins.jenkins.io/git/#plugin-content-environment-variables