Allow masterless master server builds
Allow masterless master server builds

Builds like https://github.com/elifesciences/master-server-formula/pull/1 seem to fail because the update of master servers attempts to unpin the formulas the have. In case of a masterless server, we should leave the formulas alone as one of them can be the master-server-formula being tested. In general, we can avoid unpinning these repositories.

This is a safe operation as unpinning would have failed anyway before: if this were regularly happening we would have seen it:

```
i[54.204.241.31] out: + git pull --rebase
[54.204.241.31] out: You are not currently on a branch.
[54.204.241.31] out: Please specify which branch you want to rebase against.
[54.204.241.31] out: See git-pull(1) for details.
[54.204.241.31] out:
[54.204.241.31] out:     git pull <remote> <branch>
```

elifesciences/builder,elifesciences/builder