scx.rustscheds: restore RUSTFLAGS

Per https://github.com/sched-ext/scx/blob/059ce35c31c5d7906d2886b0e1c235ef5b6f6da4/meson.build#L347-L349
but linking libbpf is not required, built in rust one should work fine.

See https://github.com/NixOS/nixpkgs/pull/437169/commits/8ec509cc139a8d5e665690fac7794ee277233f4a#r2302122465 and https://github.com/sched-ext/scx/pull/2592

pkgs/os-specific/linux/scx/scx_rustscheds.nix

···
 
   env = {
     BPF_CLANG = lib.getExe llvmPackages.clang;
+    RUSTFLAGS = lib.concatStringsSep " " [
+      "-C relocation-model=pic"
+      "-C link-args=-lelf"
+      "-C link-args=-lz"
+      "-C link-args=-lzstd"
+    ];
   };
 
   hardeningDisable = [