nix/sources.json: upgrade common to get the cargo-audit upgrade&fix
"common": {
"ref": "master",
"repo": "ssh://git@github.com/dfinity-lab/common",
- "rev": "f4ff42f4306471662f0a0124e99e4e544792b60c",
+ "rev": "7278afe2b6ddcd0c78c82fae38160bc4583fc1a6",
"type": "git"
},
"dfinity": {
