Set keep-outputs to true in all machines

From the documentation of keep-outputs, setting it to true would prevent
the GC from removing build time dependencies:

If true, the garbage collector will keep the outputs of non-garbage
derivations. If false (default), outputs will be deleted unless they are
GC roots themselves (or reachable from other roots).

In general, outputs must be registered as roots separately. However,
even if the output of a derivation is registered as a root, the
collector will still delete store paths that are used only at build time
(e.g., the C compiler, or source tarballs downloaded from the network).
To prevent it from doing so, set this option to true.

See: https://nix.dev/manual/nix/2.24/command-ref/conf-file.html#conf-keep-outputs
Reviewed-by: Aleix Roca Nonell <aleix.rocanonell@bsc.es>

m/common/base/nix.nix

@@ -23,6 +23,7 @@
       trusted-users = [ "@wheel" ];
       flake-registry = pkgs.writeText "global-registry.json"
         ''{"flakes":[],"version":2}'';
+      keep-outputs = true;
     };
 
     gc = {