build: niv common: update 2a0a8ef1 -> 4b93b09b (#987)

## Changelog for common:
Branch: master
Commits: [dfinity-lab/common@2a0a8ef1...4b93b09b](https://github.com/dfinity-lab/common/compare/2a0a8ef1f7591300ad6cfc807548376734660ace...4b93b09b796fbb62f3c4789e7527a0765eae0cdb)

* [`20f8a1f1`](https://github.com/dfinity-lab/common/commit/20f8a1f1ac78265ebaefe12d613e9931283c671f) Add ssh keys for Ryan Stout
* [`5d5d566a`](https://github.com/dfinity-lab/common/commit/5d5d566a51ce9f82e9860dec6dd65aa3a75e7402) Adding account for ianblenke
* [`bcc5333c`](https://github.com/dfinity-lab/common/commit/bcc5333c93718da7f34c01d8926877db082d7b3c) Formatting update
* [`650d5edf`](https://github.com/dfinity-lab/common/commit/650d5edf31108143a7e9ce5cda3f1667eef65f56) Update accounts.nix
* [`9be72c0b`](https://github.com/dfinity-lab/common/commit/9be72c0b2d60f35a716de0363ac7b8361e643106) nix-fmt
* [`5b45bb24`](https://github.com/dfinity-lab/common/commit/5b45bb243ef35c99074004aaee5f22ac2d98cde6) Add ssh key for Dimitris Sarlis

nix/sources.json

@@ -28,7 +28,7 @@
     "common": {
         "branch": "master",
         "repo": "ssh://git@github.com/dfinity-lab/common",
-        "rev": "2a0a8ef1f7591300ad6cfc807548376734660ace",
+        "rev": "4b93b09b796fbb62f3c4789e7527a0765eae0cdb",
         "type": "git"
     },
     "dfinity": {