more robust: avoid crash on non-Linux systems;

--- a/src/Pure/Tools/phabricator.scala	Tue Jan 23 20:50:24 2024 +0100
+++ b/src/Pure/Tools/phabricator.scala	Tue Jan 23 21:00:54 2024 +0100
@@ -202,7 +202,7 @@
   val alternative_system_port = 222
   val default_server_port = 2222
 
-  val standard_mercurial_source = {
+  def standard_mercurial_source: String = {
     val release = Linux.Release()
     if (release.is_ubuntu_20_04) "https://www.mercurial-scm.org/release/mercurial-3.9.2.tar.gz"
     else "https://www.mercurial-scm.org/release/mercurial-6.1.1.tar.gz"