build: fix JaCoCo report generation.

Prior this commit it didn't work with the following message:
[INFO] Skipping JaCoCo execution due to missing execution data file.

Regression after aa43861 (#1164)

Fix #1239

[skip ci]

Author: php-coder

pom.xml

@@ -916,8 +916,10 @@
 						Workaround for a bug in surefire plugin: it doesn't respect sourceEncoding
 						and doesn't pass it into a forked process.
 						See: https://stackoverflow.com/questions/17656475/maven-source-encoding-in-utf-8-not-working
+						
+						NB: @{argLine} is needed for JaCoCo java agent.
 					-->
-					<argLine>-Dfile.encoding=${project.build.sourceEncoding} -Xss256k</argLine>
+					<argLine>@{argLine} -Dfile.encoding=${project.build.sourceEncoding} -Xss256k</argLine>
 				</configuration>
 			</plugin>