Update JRE requires generator

Author: mizdebsk

src/main/java/org/fedoraproject/xmvn/generator/jpscript/JPackageScriptGenerator.java

@@ -28,7 +28,7 @@ public void generate(Collector collector) {
                     if (content.contains("\n. /usr/share/java-utils/java-functions\n")) {
                         collector.addRequires(filePath, "javapackages-tools");
                     }
-                    if (content.contains("\nexport JAVA_HOME=\"/usr/lib/jvm/jre-21-openjdk\"\n")) {
+                    if (content.contains("\nexport JAVA_HOME=\"${JAVA_HOME:-/usr/lib/jvm/jre-21-openjdk}\"\n")) {
                         collector.addRequires(filePath, "java-21-openjdk-headless");
                     }
                 }

src/test/resources/usr/bin/jflex

@@ -4,7 +4,7 @@
 # JPackage Project <http://www.jpackage.org/>
 
 # Set default JAVA_HOME
-export JAVA_HOME="/usr/lib/jvm/jre-21-openjdk"
+export JAVA_HOME="${JAVA_HOME:-/usr/lib/jvm/jre-21-openjdk}"
 
 # Source functions library
 . /usr/share/java-utils/java-functions