Though I haven't tested that yet, I don't think Run#getEnvironment(TaskListener) resolved the problem as it doesn't extract variables from ParametersAction.
I think I have to resolve this in a following way:

• Make ParameterizedBuildSelector accepts also immediate values.
  • It's good to implement this as a fallback when failed to resolve the variable.
• Update the configuration to pass not the variable name (VARNAME) but the variable value ${VARNAME)

I don't think Run#getEnvironment(TaskListener) resolved the problem as it doesn't extract variables from ParametersAction

Ah, true, this code is located elsewhere. Still, the code I pointed to should be fixed, it would just not help in this case. So actually ParameterizedBuildSelector ought to ignore env and instead ask parent for ParametersAction. (Currently ParametersAction.buildEnvVars does not offer a Run overload but it can be trivially inlined, as I do in the code above.) And of course it should gracefully handle the case that there is no such parameter; rather than throwing an NPE, throw a meaningful exception, or return null. (Fallbacks here are a poor idea since parameters already have default values.)

Anyway there is no purpose in using ParameterizedBuildSelector from a Workflow to begin with that I can see. Use SpecificBuildSelector and use logic in your script to pass a desired number rather than a string constant.

https://github.com/jenkinsci/copyartifact-plugin/pull/70

Code changed in jenkins
User: ikedam
Path:
src/test/java/hudson/plugins/copyartifact/ParameterizedBuildSelectorTest.java
http://jenkins-ci.org/commit/copyartifact-plugin/bab56aae014bf259755fdb3d966dac0be35382b3
Log:
JENKINS-30357 Added tests to reproduce JENKINS-30357, NPE when ParameterizedBuildSelector in workflow, or using an undefined variable.

Code changed in jenkins
User: ikedam
Path:
src/main/java/hudson/plugins/copyartifact/CopyArtifact.java
src/main/java/hudson/plugins/copyartifact/ParameterizedBuildSelector.java
http://jenkins-ci.org/commit/copyartifact-plugin/6ddc88a0e0609bde83d8aecbd90170f254cc8fcf
Log:
JENKINS-30357 Fixed NPE when using ParameterizedBuildSelector in Workflow or using undefined variables in ParameterizedBuildSelector.

Code changed in jenkins
User: ikedam
Path:
src/main/java/hudson/plugins/copyartifact/ParameterizedBuildSelector.java
src/test/java/hudson/plugins/copyartifact/ParameterizedBuildSelectorTest.java
http://jenkins-ci.org/commit/copyartifact-plugin/4e6cb78a2226ffe238a25ddfa788866c8cb4f131
Log:
[FIXED JENKINS-30357] Improved error handlings in ParameterizedBuildSelector.

Code changed in jenkins
User: ikedam
Path:
src/main/java/hudson/plugins/copyartifact/ParameterizedBuildSelector.java
src/main/resources/hudson/plugins/copyartifact/ParameterizedBuildSelector/help-parameterName.html
src/test/java/hudson/plugins/copyartifact/ParameterizedBuildSelectorTest.java
http://jenkins-ci.org/commit/copyartifact-plugin/43699c2f605172202f228d627a4cb329dc95f984
Log:
JENKINS-30357 Extends ParameterizedBuildSelector to also accepts immediate values (<Selector />) and variable expressions (${SELECTOR}).

Code changed in jenkins
User: ikedam
Path:
src/test/java/hudson/plugins/copyartifact/ParameterizedBuildSelectorTest.java
http://jenkins-ci.org/commit/copyartifact-plugin/d7df521bde6fe4dae57ad16c962e1957435b70e1
Log:
JENKINS-30357 Fixed failing tests.

Code changed in jenkins
User: ikedam
Path:
src/main/java/hudson/plugins/copyartifact/CopyArtifact.java
src/main/java/hudson/plugins/copyartifact/ParameterizedBuildSelector.java
src/main/resources/hudson/plugins/copyartifact/ParameterizedBuildSelector/help-parameterName.html
src/test/java/hudson/plugins/copyartifact/ParameterizedBuildSelectorTest.java
http://jenkins-ci.org/commit/copyartifact-plugin/b72f3f9c95d656ff72ead97ffce7e94684d38571
Log:
Merge pull request #70 from ikedam/feature/JENKINS-30357_NpeInParameterizedBuildSelector

JENKINS-30357 Fixed NPE and other improvements in ParameterizedBuildSelector

Compare: https://github.com/jenkinsci/copyartifact-plugin/compare/f4946461e5f4...b72f3f9c95d6

Fixed in copyartifact-1.36.1.
It will be available in the update center in a day.
Please try that.