Add JAVA15_HOME environment variable for Windows pull request builds

Author: mark-vieira

.ci/jobs.t/elastic+elasticsearch+pull-request+part-1-windows.yml

@@ -33,6 +33,7 @@
           properties-content: |
             JAVA_HOME=$USERPROFILE\\.java\\$ES_BUILD_JAVA
             RUNTIME_JAVA_HOME=$USERPROFILE\\.java\\$ES_RUNTIME_JAVA
+            JAVA15_HOME=$USERPROFILE\\.java\\openjdk15
             GRADLE_TASK=checkPart1
       - batch: |
           del /f /s /q %USERPROFILE%\.gradle\init.d\*.*

.ci/jobs.t/elastic+elasticsearch+pull-request+part-2-windows.yml

@@ -33,6 +33,7 @@
           properties-content: |
             JAVA_HOME=$USERPROFILE\\.java\\$ES_BUILD_JAVA
             RUNTIME_JAVA_HOME=$USERPROFILE\\.java\\$ES_RUNTIME_JAVA
+            JAVA15_HOME=$USERPROFILE\\.java\\openjdk15
             GRADLE_TASK=checkPart2
       - batch: |
           del /f /s /q %USERPROFILE%\.gradle\init.d\*.*