Hardcode Racket variant to CS

Hardcode variant detection because nixpkgs wraps the Racket binary making it
fail to detect its variant at runtime.
https://github.com/NixOS/nixpkgs/issues/114993#issuecomment-812951247

--- old/collects/setup/variant.rkt
+++ new/collects/setup/variant.rkt
@@ -7,7 +7,8 @@
 (provide variant-suffix
          script-variant?)
 
-(define plain-variant
+(define plain-variant 'cs)
+#;(define plain-variant
   (delay/sync
    (cond
      [(cross-installation?)