When bootstrapping, check the user's namespace first with their own branch before falling back on matrix-org

[ShadowJonathan]

sytest/docker/bootstrap.sh

Line 32 in cfbf743

wget -q https://github.com/matrix-org/sytest/archive/$branch_name.tar.gz -O sytest.tar.gz || {

Edit that to first check user_repo/sytest@user_branch, before going to matrix-org/sytest@user_branch and then matrix-org/sytest@develop

[richvdh]

I have a feeling we avoided this because "security" (something to do with untrusted code in CI). But possibly that was pre-GHA, and is no longer a problem?

handwave. I can't really remember.

[ShadowJonathan]

Maybe it could still become a security concern if/when matrix.org adds their own self-hosted runners to the GHA pool, though maybe that could be re-routed for untrusted PRs?