Fix Sytest job failing on daily Twisted trunk workflow

[anoadragon453]

Addresses the first portion of #20292. See that issue for the full context.

Broke in #19234.

The run now succeeds when run against this PR's branch: https://github.com/element-hq/synapse/actions/runs/36712766021

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[MadLittleMods]

Why did we ever use OFFLINE: 1 if it isn't necessary?

[anoadragon453]

Looks like setting this option was actually done unnecessarily by matrix-org/synapse#12425. Doing so was useful in an earlier draft of matrix-org/synapse#12337 (which 12425 was extracted from). That earlier draft contained:

poetry install --no-interaction --extras "all test"
poetry run pip install git+https://github.com/twisted/twisted.git@trunk

in the .ci/patch_for_twisted_trunk.sh script.

This installs the normal locked dependencies via poetry, then replaced Twisted using pip. It did not update pyproject.toml or poetry.lock. The reason given in the script was:

# ... we run into https://github.com/python-poetry/poetry/issues/5311, where
# poetry insists on installing an old version of treq, which isn't actually compatible
# with recent twisted releases. So let's just install twisted trunk using pip.

pip was used to install Twisted trunk instead of poetry. With that, running poetry install again would override the Twisted version pip installed. Thus, OFFLINE: 1 was used to prevent that second poetry install.

Later, the pip invocation was removed, and replaced with poetry install again. But the OFFLINE: 1 was left over (and unnecessary).

The OFFLINE switch itself originated from matrix-org/sytest#588. So we may still want to keep it, even if no CI references it anymore; if we deem the use case worthwhile.

[anoadragon453]

Re-ran the Twisted Trunk job on this branch and it still succeeds: https://github.com/element-hq/synapse/actions/runs/37448667754

Thanks for the review @MadLittleMods!