ci: install unsloth_zoo from git main in notebooks-ci + studio-backend-ci (#5802)

* ci: install unsloth_zoo from git main in notebooks-ci + studio-backend-ci

These were the only two workflows that still pulled unsloth_zoo from
PyPI; every other CI (Core, MLX, version-compat, install.sh-driven
Studio smokes) installs zoo from git main. Drift between PyPI and
main hides fixes-on-zoo-main and lets PR-time validation pass on a
stale zoo, then break for users on next release.

Both edits match the retry-with-backoff shape mlx-ci.yml already uses.

* ci: drop --no-deps from studio-backend-ci unsloth_zoo install

The prior PyPI line was `pip install 'unsloth_zoo>=2026.5.1'` (no
--no-deps), which pulled in triton and the rest of zoo's runtime deps.
I dropped that transitive resolve in the first commit, which broke
collection of 5 tests in Repo tests (CPU) with
ModuleNotFoundError: No module named 'triton'.

Match the prior dep-resolve shape, keeping the source-from-git change.
notebooks-ci keeps --no-deps because its original line also had it.