lib/systems: fix scaleway-c1 platform

This regressed in 9c213398b3

The recursiveUpdate gave the platform both gcc.cpu and gcc.arch attrs
instead of only gcc.cpu. This is invalid; gcc configuration fails with:

```
Switch "--with-arch" may not be used with switch "--with-cpu"
```

So we revert to using `//` to retain only gcc.cpu
(which is more specific than the processor arch).

lib/systems/platforms.nix

@@ -233,7 +233,7 @@ rec {
     };
   };
 
-  scaleway-c1 = lib.recursiveUpdate armv7l-hf-multiplatform {
+  scaleway-c1 = armv7l-hf-multiplatform // {
     gcc = {
       cpu = "cortex-a9";
       fpu = "vfpv3";