[2.0.x] fix: Honor JAVA_HOME in the Unix launcher (#9825)

Problem: The Unix launcher ignores JAVA_HOME and selects Java from PATH when JAVACMD is absent.

Solution: Check JAVA_HOME/bin/java before falling back to PATH while preserving explicit --java-home and JAVACMD precedence.

Fixes #9794

launcher-package/integration-test/src/test/scala/ShellScriptUtil.scala

@@ -186,6 +186,7 @@ trait ShellScriptUtil extends BasicTestSuite {
           if (isWindows)
             envVars("JAVACMD") = new File(javaBinDir, "java").getAbsolutePath()
           else
+            envVars("JAVACMD") = new File(javaBinDir, "java").getAbsolutePath()
             envVars("PATH") = javaBinDir + File.pathSeparator + path
 
           val cmd = LauncherTestHelper.launcherCommand(testSbtScript.getAbsolutePath) ++ args

sbt

@@ -927,6 +927,12 @@ original_args=("$@")
 
 sbt_file_opts=()
 
+if [[ -f "$JAVACMD" ]]; then
+  java_cmd="$JAVACMD"
+elif [[ -n "$JAVA_HOME" && -f "$JAVA_HOME/bin/java" ]]; then
+  java_cmd="$JAVA_HOME/bin/java"
+fi
+
 # Pull in the machine-wide settings configuration.
 if [[ -f "$machine_sbt_opts_file" ]]; then
   sbt_file_opts+=($(loadConfigFile "$machine_sbt_opts_file"))