Currently the P4 counter text field does not expand env variables when run.

Steps to reproduce:
1) Setup a build job
2) Enable Perforce plugin
3) In the P4 Counter text field, enter a counter name with env varaible, example: counter_${JOB_NAME}
4) Start the build

Result:
The perforce plugin saves the synced chagnelist to counter_${JOB_NAME} instead of substituting it when creating a counter name. Env substitution would be useful in here as I can set this once and use it in different jobs.

I have actually re-compiled the latest 1.3.8-snapshot with changes to support env variables substitution. It was a one line change...what would be best method to publish my work? (Sorry this is my first time contributing