opensmalltalk-vm: don't throw without aliases

CI needs ugly workarounds to catch a `throw` on unsupported platforms.
If aliases are disabled, as is the case in CI, we simply return `null`
for this leaf package. This will ignore it for CI purposes, while still
giving a proper error message for end users.

pkgs/development/compilers/opensmalltalk-vm/default.nix

@@ -1,4 +1,5 @@
 {
+  config,
   stdenv,
   lib,
   fetchFromGitHub,
@@ -190,7 +191,11 @@ let
 
   platform = stdenv.targetPlatform.system;
 in
-vmsByPlatform.${platform} or (throw (
-  "Unsupported platform ${platform}: only the following platforms are supported: "
-  + builtins.toString (builtins.attrNames vmsByPlatform)
-))
+if (!config.allowAliases && !(vmsByPlatform ? platform)) then
+  # Don't throw without aliases to not break CI.
+  null
+else
+  vmsByPlatform.${platform} or (throw (
+    "Unsupported platform ${platform}: only the following platforms are supported: "
+    + builtins.toString (builtins.attrNames vmsByPlatform)
+  ))