Add missing requirements for sharktank t5 tests. (#788)

Fixes #787. See the workflow
passing now:
https://github.com/nod-ai/shark-ai/actions/runs/12676783895/job/35330696315?pr=788.

We could also install another package that transitively includes
`protobuf`, like `transformers[sentencepiece]` or `onnx`. I'm also not
sure if this should be a test requirement or a release requirement,
given how limited and indirect the usage is, so just starting simple and
explicit.

sharktank/requirements-tests.txt

@@ -1,6 +1,7 @@
 datasets==3.0.0
 diffusers
 parameterized
+protobuf
 pytest==8.0.0
 pytest-html
 pytest-xdist==3.5.0