Github Actions: solid-test-suite fails on PRs from forked repo

[angelo-v]

When running the github actions build on a pull request from a fork, it will fail at "Run bash test/surface/run-solid-test-suite.sh $BRANCH_NAME" because it assumes a branch in the local repository, but it is on the fork only

See: https://github.com/solid/node-solid-server/runs/4895773683?check_suite_focus=true#step:13:70

error: pathspec 'docker-compose' did not match any file(s) known to git

(docker-compose is the branch name in the fork in this case)