test: force protobuf 25.5 on v1.37.0

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for test only.

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kokoro integration test fails with

fatal: ambiguous argument '1.37.x...936ef89d0a66da68d200b496865edad7e9000052': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git  [...] -- [...]'

Seems test setup does not deal correctly when trying to merge to branch.

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kokoro integration failures on dataproc, compute and vertexai. Not in scope for lts.