Use BUILD_COUNTER instead of BUILD_NUMBER

* BUILD_NUMBER is redefined by this build file on CI so we cannot run more
  than one Gradle step.
* BUILD_COUNTER is defined on TeamCity to be %build.counter% and is not
  touched by the build.

Author: sergej-koscejev

build.gradle

@@ -81,7 +81,7 @@ if (project.hasProperty('iets3OpenSourceVersion')) {
     if (ciBuild) {
         currentBranch = GitBasedVersioning.gitBranch
 
-        def buildNumber = System.env.BUILD_NUMBER.toInteger()
+        def buildNumber = System.env.BUILD_COUNTER.toInteger()
         if (currentBranch.startsWith("maintenance-")) {
             version = "$major.$minor.$buildNumber.${GitBasedVersioning.gitShortCommitHash}"
         } else {