bugfix: Don't try to always get system jvm first

modules/core/src/main/scala/scala/build/internals/OsLibc.scala

@@ -74,14 +74,7 @@ object OsLibc {
   }
 
   def defaultJvm(os: String): String = {
-    val hasEmptyJavaHome = Option(System.getenv("JAVA_HOME")).exists(_.trim.isEmpty)
-    val defaultJvm0      = baseDefaultJvm(os, defaultJvmVersion)
-    if (hasEmptyJavaHome)
-      // Not using the system JVM if JAVA_HOME is set to an empty string
-      // (workaround for https://github.com/coursier/coursier/issues/2292)
-      defaultJvm0
-    else
-      s"${JavaHome.systemId}|$defaultJvm0"
+    baseDefaultJvm(os, defaultJvmVersion)
   }
 
   def javaVersion(javaCmd: String): Int = {

modules/integration/src/test/scala/scala/cli/integration/StandaloneLauncherTests.scala

@@ -6,26 +6,28 @@ import java.io.File
 
 class StandaloneLauncherTests extends ScalaCliSuite {
 
-  if (TestUtil.isJvmCli)
-    test(s"Standalone launcher should run with java 8") {
-      // It should download Java 17 and use it to run itself
-      val message = "Hello World"
-      val inputs = TestInputs(
-        os.rel / "hello.sc" -> s"""println("$message")"""
+  test(s"Standalone launcher should run with java 8") {
+    // It should download Java 17 and use it to run itself
+    val message = "Hello World"
+    val inputs = TestInputs(
+      os.rel / "hello.sc" -> s"""println("$message")"""
+    )
+    inputs.fromRoot { root =>
+      val java8Home =
+        os.Path(os.proc(TestUtil.cs, "java-home", "--jvm", "zulu:8").call().out.trim(), os.pwd)
+
+      val extraEnv = Map(
+        "JAVA_HOME" -> java8Home.toString,
+        "PATH"      -> ((java8Home / "bin").toString + File.pathSeparator + System.getenv("PATH"))
       )
-      inputs.fromRoot { root =>
-        val java8Home =
-          os.Path(os.proc(TestUtil.cs, "java-home", "--jvm", "zulu:8").call().out.trim(), os.pwd)
 
-        val extraEnv = Map(
-          "JAVA_HOME" -> java8Home.toString,
-          "PATH"      -> ((java8Home / "bin").toString + File.pathSeparator + System.getenv("PATH"))
-        )
+      // we need to exit to check if the launcher is actually starting the right version of java
+      os.proc(TestUtil.cli, "--power", "bloop", "exit").call(cwd = root, env = extraEnv).out.trim()
 
-        val output =
-          os.proc(TestUtil.cli, ".").call(cwd = root, env = extraEnv).out.trim()
+      val output =
+        os.proc(TestUtil.cli, ".").call(cwd = root, env = extraEnv).out.trim()
 
-        expect(output == message)
-      }
+      expect(output == message)
     }
+  }
 }

modules/options/src/main/scala/scala/build/options/BuildOptions.scala

@@ -557,7 +557,7 @@ object BuildOptions {
     def envUpdates(currentEnv: Map[String, String]): Map[String, String] = {
       // On Windows, AFAIK, env vars are "case-insensitive but case-preserving".
       // If PATH was defined as "Path", we need to update "Path", not "PATH".
-      // Same for JAVA_HOME.
+      // Same for JAVA_HOME
       def keyFor(name: String) =
         if (Properties.isWin)
           currentEnv.keys.find(_.equalsIgnoreCase(name)).getOrElse(name)

modules/options/src/main/scala/scala/build/options/JavaOptions.scala

@@ -149,7 +149,7 @@ final case class JavaOptions(
     }
 
   private def findLocalDefaultJava(): Option[Positioned[os.Path]] =
-    Option(System.getenv("JAVA_HOME")).map(p =>
+    Option(System.getenv("JAVA_HOME")).filter(_.nonEmpty).map(p =>
       Positioned(Position.Custom("JAVA_HOME env"), os.Path(p, os.pwd))
     ).orElse(
       sys.props.get("java.home").map(p =>