nix: upgrade common to fix the cargo-security-audit job (#272)

Previously `cargo-security-audit` didn't produce the required $out when run
successfully. Now the $out produced is the output from `cargo audit`.

The output is also registered as a Hydra build product so that we can
directly see the report from Hydra. The latter is useful for checking
which warnings were reported.

nix/default.nix

@@ -22,7 +22,7 @@ let
     else builtins.fetchGit {
       name = "common-sources";
       url = "ssh://[email protected]/dfinity-lab/common";
-      rev = "a066833f9ce8fac453f736639d46021a714682b2";
+      rev = "7fc0d40dc6e52a221b370be5a04292e59b32f17d";
     };
 in import commonSrc {
   inherit system crossSystem config;