Update actions/cache@v2, which is soon to be retired by Github actions

[jafingerhut]

No description provided.

[jafingerhut]

According to a log message in a failing test for this recently created PR that modifies one of the .proto files, the Github action actions/cache@v2 used by ci-build-proto.yml is soon to be retired (2025-Mar-01), and is being failed intermittently before then as a warning to repos still using it:

Error: This request has been automatically failed because it uses a deprecated version of `actions/cache: v2`. Please update your workflow to use v3/v4 of actions/cache to avoid interruptions. Learn more: https://github.blog/changelog/2024-12-05-notice-of-upcoming-releases-and-breaking-changes-for-github-actions/#actions-cache-v1-v2-and-actions-toolkit-cache-package-closing-down

[jafingerhut]

@chrispsommers @smolkaj By 2025-Mar-01, Github says that our current CI build task for compiling modified .proto files will fail deterministically as it is currently written. It fails intermittently now, as a warning.

I am not certain that this PR is the best way to improve the situation, but it seems recommended by Github.

[chrispsommers]

LGTM, let's have @smolkaj take a look.

[smolkaj]

Thanks!