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

Previously, we would always first specify `system|` when downloading default JVM, which would coursier just return java from JAVA_HOME without any checks even if we needed to download never Java. Now, we only use specific versions, since java home is handled before.

Author: tgodzik

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)
     }
+  }
 }