Steps to reproduce:
1. Set an environment variable in global configuration for example GIT_URL=https://some/url/$PROJECT
2. Set parameter of PROJECT=netvirt for example
3. Create a job that calls "inject" setting a different parameter for example "VAR=123" and then in a shell step immediately after "echo $GIT_URL".
4. Run job
GIT_URL is overrided by inject to be a literal string of '$GIT_URL/netvirt/netvirt/netvirt/netvirt'
The inject plugin should not be overriding existing variables that are not passed in during the inject call.