https://issues.jenkins-ci.org/browse/JENKINS-16316

slave (which are outdated if you dont restart slave nodes after changing
global properties)

CAUTION - this might break some features: EnvironmentContributors don't
get recognized any more (because CoreEnvironmentContributor did a
Computer.current.getEnvironment() which also leads to overwriting with
outdated values.

how to reproduce the problem:
hpi:run on current head with jenkins-version set to "1.480.3" (current LTS verison)
configure a global property on master K/V: TESTVAR/value1
configure a slave and start it
configure a job restricted to run on that slave and to echo $TESTVAR.
run the job (it will echo "value1")
configure the global property on master to be: TESTVAR/value2
dont restart anything
rerun the job (it will still echo "value1")
however a job on master will recognize the changed value.
there are 2 pieces of code that lead to this behaviour. I'm pretty uncertain about the fix i made, but it resolves at least our problem with the plugin as i described it above, although i'm pretty sure it breaks some other intended feature.
https://github.com/kazesberger/envinject-plugin.git
https://github.com/kazesberger/envinject-plugin/commits/master
maybe someone can have a look at it and/or tell me what's the story behind this (Core)EnvironmentContributor thing.