Update friendbot source to be new repository (#817)

### What
Update friendbot image configuration to reference the stellar/friendbot
repository instead of stellar/go for the nightly images.

  ### Why
Friendbot has been extracted into its own repository, requiring updated
image sources for nightly builds because they build off the latest code.

The latest, testing, and futurenet builds will be updated separately.

For stellar/friendbot#4

Author: leighmcculloch

images.json

@@ -72,7 +72,7 @@
       { "name": "core", "repo": "stellar/stellar-core", "ref": "master", "options": { "configure_flags": "--disable-tests" } },
       { "name": "rpc", "repo": "stellar/stellar-rpc", "ref": "main" },
       { "name": "horizon", "repo": "stellar/go", "ref": "master" },
-      { "name": "friendbot", "repo": "stellar/go", "ref": "master" },
+      { "name": "friendbot", "repo": "stellar/friendbot", "ref": "main" },
       { "name": "lab", "repo": "stellar/laboratory", "ref": "main" }
     ],
     "tests": {
@@ -92,7 +92,7 @@
       { "name": "core", "repo": "stellar/stellar-core", "ref": "master", "options": { "configure_flags": "--disable-tests --enable-next-protocol-version-unsafe-for-production" } },
       { "name": "rpc", "repo": "stellar/stellar-rpc", "ref": "main" },
       { "name": "horizon", "repo": "stellar/go", "ref": "master" },
-      { "name": "friendbot", "repo": "stellar/go", "ref": "master" },
+      { "name": "friendbot", "repo": "stellar/friendbot", "ref": "main" },
       { "name": "lab", "repo": "stellar/laboratory", "ref": "main" }
     ],
     "tests": {