Use -Dsbt.script to start sbt server

In order to start the sbt server we must use the sbt script because
it is the only way to load the .sbtopts and the .jvmopts file properly.

To do so the sbt script can pass a -Dsbt.script prop to the java server.
It is used in the NetworkClient to start the server, and it is replicated
in the BuildServerConnection file (.bsp/sbt.json).

main-command/src/main/scala/sbt/internal/client/NetworkClient.scala

@@ -1045,7 +1045,8 @@ object NetworkClient {
   private[client] val noStdErr = "--no-stderr"
   private[client] val sbtBase = "--sbt-base-directory"
   private[client] def parseArgs(args: Array[String]): Arguments = {
-    var sbtScript = if (Properties.isWin) "sbt.bat" else "sbt"
+    val defaultSbtScript = if (Properties.isWin) "sbt.bat" else "sbt"
+    var sbtScript = Properties.propOrNone("sbt.script")
     var launchJar: Option[String] = None
     var bsp = false
     val commandArgs = new mutable.ArrayBuffer[String]
@@ -1067,11 +1068,10 @@ object NetworkClient {
           sbtScript = a
             .split("--sbt-script=")
             .lastOption
-            .map(_.replaceAllLiterally("%20", " "))
-            .getOrElse(sbtScript)
+            .orElse(sbtScript)
         case "--sbt-script" if i + 1 < sanitized.length =>
           i += 1
-          sbtScript = sanitized(i).replaceAllLiterally("%20", " ")
+          sbtScript = Some(sanitized(i))
         case a if a.startsWith("--sbt-launch-jar=") =>
           launchJar = a
             .split("--sbt-launch-jar=")
@@ -1091,12 +1091,17 @@ object NetworkClient {
     }
     val base = new File("").getCanonicalFile
     if (!sbtArguments.contains("-Dsbt.io.virtual=true")) sbtArguments += "-Dsbt.io.virtual=true"
+    if (!sbtArguments.exists(_.startsWith("-Dsbt.script"))) {
+      sbtScript.foreach { sbtScript =>
+        sbtArguments += s"-Dsbt.script=$sbtScript"
+      }
+    }
     new Arguments(
       base,
       sbtArguments.toSeq,
       commandArgs.toSeq,
       completionArguments.toSeq,
-      sbtScript,
+      sbtScript.getOrElse(defaultSbtScript).replaceAllLiterally("%20", " "),
       bsp,
       launchJar
     )