Fix build CI in fork contexts

[namoscato]

i.e. in #234, actions/checkout fails, presumably due to the ref input:

Fetching the repository
  /usr/bin/git -c protocol.version=2 fetch --no-tags --prune --progress --no-recurse-submodules --depth=1 origin +refs/heads/readme-note-limitations*:refs/remotes/origin/readme-note-limitations* +refs/tags/readme-note-limitations*:refs/tags/readme-note-limitations*
  The process '/usr/bin/git' failed with exit code 1
  Waiting 15 seconds before trying again
  /usr/bin/git -c protocol.version=2 fetch --no-tags --prune --progress --no-recurse-submodules --depth=1 origin +refs/heads/readme-note-limitations*:refs/remotes/origin/readme-note-limitations* +refs/tags/readme-note-limitations*:refs/tags/readme-note-limitations*
  The process '/usr/bin/git' failed with exit code 1
  Waiting 15 seconds before trying again
  /usr/bin/git -c protocol.version=2 fetch --no-tags --prune --progress --no-recurse-submodules --depth=1 origin +refs/heads/readme-note-limitations*:refs/remotes/origin/readme-note-limitations* +refs/tags/readme-note-limitations*:refs/tags/readme-note-limitations*
  Error: The process '/usr/bin/git' failed with exit code 1