[JENKINS-26583] - Attempt to tweak the injection priorities

[oleg-nenashev]

It is an exploratory attempt to fix the resolution order and to make tests provided by @olivergondza working. The fix in #37 was not working with the recent versions of EnvInject (due to #56 by @recena and other changes).

• - Integrated [WIP][JENKINS-26583] Env-inject suppress variables contributed by extension points #37 with master, polished tests a bit
• - Prevented EnvInjectPluginAction from overriding variables on late stages, 3 tests fixed
• - Change the priority of EnvInjectListener to call it first and to let others override the EnvVars
• - Stop overriding already defined variables in EnvInject listener (likely a breaking change)
• [ ]
  ....
• - Make it actually working

@reviewbybees @olivergondza @recena

[ghost]

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