[2.x] fix: hash the boot classpath in a deterministic order (#9809)

The launcher enumerates the boot directory, so `AppProvider.mainClasspath` and `ScalaProvider.jars` come back
in filesystem order, and sbt 2 folds that classpath into the metabuild's `CompileInputs2`. Two machines with
identical inputs therefore compute different keys: measured on one project at the same commit, same sbt and
same JDK, 439 classpath entries on both sides, the same set, 83 of them at a different index and all under
${SBT_BOOT}.

Sorting happens after conversion to the rooted virtual path, so the order is independent of the filesystem
and of where the boot directory lives. The order being replaced is a directory listing, not one anybody
chose. `Main.rawEval` builds the same list but never reaches a cache key, so it is left alone.

Generated-by: Claude Opus 5 (Claude Code)

main/src/main/scala/sbt/Defaults.scala

@@ -4702,9 +4702,11 @@ object Classpaths:
     val app = appConfiguration.value
     def isJansiOrJLine(f: File) = f.getName.contains("jline") || f.getName.contains("jansi")
     val scalaInstanceJars = app.provider.scalaProvider.jars.filterNot(isJansiOrJLine)
+    // Sorted for the same reason as the metabuild classpath in `Load`.
     val sbtCp = (scalaInstanceJars ++ app.provider.mainClasspath)
       .map(_.toPath)
       .map(p => converter.toVirtualFile(p): HashedVirtualFileRef)
+      .sortBy(_.id)
       .map(Attributed.blank)
     val cpConfig = classpathConfiguration.value
     val up = update.value

main/src/main/scala/sbt/internal/Load.scala

@@ -100,8 +100,9 @@ private[sbt] object Load:
     val loader = getClass.getClassLoader
     val converter = MappedFileConverter(rootPaths, true)
     val cp0 = provider.mainClasspath.toIndexedSeq ++ scalaProvider.jars.toIndexedSeq
+    // The launcher's order is a directory listing, and this classpath is hashed into the compile key.
     val classpath = Attributed.blankSeq(
-      cp0.map(_.toPath).map(p => converter.toVirtualFile(p): HashedVirtualFileRef)
+      cp0.map(_.toPath).map(p => converter.toVirtualFile(p): HashedVirtualFileRef).sortBy(_.id)
     )
     val csrConfig = CoursierConfiguration()
       .withResolvers(Resolver.combineDefaultResolvers(Vector.empty).toVector)