[2.x] ci: Handle undefined crossScalaVersions in publishLocalAllModule (#8491)

The publishLocalAllModule command fails with 'crossScalaVersions is
undefined' when the zinc submodule's compilerBridge project doesn't
have crossScalaVersions defined.

This is a long-standing issue that was reported in #4387 (2018) and
'magically resolved' when zinc happened to define the setting, then
resurfaced in #7607 (2024) when zinc changed.

The fix uses getOpt().getOrElse(Nil).toList instead of get().toList
to handle the case where crossScalaVersions is not defined, allowing
the command to proceed gracefully.

Fixes #7607

build.sbt

@@ -1060,7 +1060,8 @@ def customCommands: Seq[Setting[?]] = Seq(
       }).toList :::
       (zincOpt map { case ProjectRef(build, _) =>
         val zincSv = get((ProjectRef(build, "zinc") / scalaVersion))
-        val csv = get((ProjectRef(build, "compilerBridge") / crossScalaVersions)).toList
+        val csv =
+          getOpt((ProjectRef(build, "compilerBridge") / crossScalaVersions)).getOrElse(Nil).toList
         (csv flatMap { bridgeSv =>
           s"++$bridgeSv" :: ("{" + build.toString + "}compilerBridge/publishLocal") :: Nil
         }) :::