Mention publish-local-dev custom command in contributors' doc (#3277)

There's no longer a special scripts like `scripts/publish-local`.
ScalaNative is required to depend on multiple Scala versions
e.g. Scala 2.12 (sbtPlugin) or Scala 2.13 (scalalib for Scala 3)
and just running `++x.y.z publishLocal` on root project is not
recommended.
Instead, we can use `publish-local-dev` custom task in sbt.

see: #3270 (review)
(cherry picked from commit 713593a)

docs/contrib/build.rst

@@ -140,11 +140,11 @@ If you need to test your copy of Scala Native in the larger context of a
 separate build, you will need to locally publish all the artifacts of Scala
 Native.
 
-Use the special script that publishes all the cross versions:
+Use the custom sbt command to publish all projects for a specific Scala version (`x,y,z`):
 
 .. code-block:: text
 
-    $ scripts/publish-local
+    > publish-local-dev x.y.z
 
 Afterwards, set the version of `sbt-scala-native` in the target project's
 `project/plugins.sbt` to the current SNAPSHOT version of Scala Native, and use