Cannot run auto format on a fork

[xross]

It seems auto format cannot be run when PR'ing a branch from a fork, only when a PR is merging an upstream branch.

for example, I have a fix branch, say on fix/101 on my fork. I get the following error as it tried to checkout the branch on the upstream repo, which doesn't exist:

/usr/bin/git checkout fix/101
[72](https://github.com/xmos/lib_xud/runs/5424431483?check_suite_focus=true#step:3:72)
error: pathspec 'fix/101' did not match any file(s) known to git