Make GHA job triggers predictable.

AFAIK Jenkins jobs have up to three different names.
 
1. The job name stem, e.g. "PostCommit_Java_VR_Dataflow_V2". This is defined in the source file "job_PostCommit_Java_ValidatesRunner_Dataflow_V2.groovy" (I assume the "job_" prefix is arbitrary; I'm not sure why we use a filename prefix instead of a proper subfolder). In Jenkins, the job name is prefixed by "beam_" (I assume for historical reasons) on Jenkins and postfixed for variants like "_cron" or "_phrase". 
2. The Github comment trigger, e.g. "Run Java Dataflow V2 ValidatesRunner"
3. The job name displayed on Github, e.g. "Google Cloud Dataflow Runner V2 Java ValidatesRunner Tests"
 
The latter two seem redundant.
 

Imported from Jira [BEAM-12412](https://issues.apache.org/jira/browse/BEAM-12412). Original Jira may contain additional context.
Reported by: ibzib.