[FIXED JENKINS-19926] Add only new env. variables

Previous patch (3eea39648943583a530f47f69f93737266537d9d) broke usescases when evn. variable overwrites already existing one
like PATH = $PATH:/my/path
Merge pull request #970 from vjuranek/JENKINS-19926

[FIXED JENKINS-19926] Add only new env. variables