test: share NixOS-aware Postgres bootstrap between both EmbeddedPostgres instances

Test 9 was started with a bare EmbeddedPostgres.start(), bypassing the
NOMAD_PG_TARBALL resolver applied to the first instance. On NixOS, the
fallback to zonky's bundled generic-Linux binaries failed with the
dynamic-linker error from stub-ld. Extract the bootstrap into
startEmbeddedPostgres() and route both instances through it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: NovaMage

sbt-plugin/src/sbt-test/nomad/clean-and-migrate-postgres/src/main/scala/Main.scala

@@ -2,11 +2,11 @@ import nomad.{Migrator, SQLMigration, SupportedDatabase}
 import io.zonky.test.db.postgres.embedded.EmbeddedPostgres
 
 object Main {
-  def main(args: Array[String]): Unit = {
-    // On NixOS zonky's bundled generic-Linux binaries fail to load. NOMAD_PG_TARBALL
-    // (set by the flake devshell) overrides the binary source with a tarball of the
-    // system postgres.
-    val pg = sys.env.get("NOMAD_PG_TARBALL").filter(_.nonEmpty) match {
+  // On NixOS zonky's bundled generic-Linux binaries fail to load. NOMAD_PG_TARBALL
+  // (set by the flake devshell) overrides the binary source with a tarball of the
+  // system postgres.
+  private def startEmbeddedPostgres(): EmbeddedPostgres =
+    sys.env.get("NOMAD_PG_TARBALL").filter(_.nonEmpty) match {
       case Some(path) =>
         // nix-store postgres defaults unix_socket_directories to /run/postgresql,
         // which doesn't exist in this sandbox — point it at the JVM temp dir instead.
@@ -18,6 +18,9 @@ object Main {
       case None => EmbeddedPostgres.start()
     }
 
+  def main(args: Array[String]): Unit = {
+    val pg = startEmbeddedPostgres()
+
     try {
       val ds = pg.getPostgresDatabase()
       val migrations = Vector(SQLMigration("migrations/M001_CreateUsers.sql"))
@@ -218,7 +221,7 @@ object Main {
       // additionally verifies that the fix is idempotent: a second cleanAndMigrate() call
       // after self-heal must also converge (the bug was described as self-perpetuating).
       // Use a fresh embedded PG so earlier tests do not interfere with the public schema.
-      val pg2 = EmbeddedPostgres.start()
+      val pg2 = startEmbeddedPostgres()
       try {
         val ds2 = pg2.getPostgresDatabase()