test: build local spread binary

[cmatsuoka]

Don't rely on GitHub runners providing a (potentially outdated)
spread binary. Instead, check out a reference version and build a
local binary to be used in the github CI test.

[Copilot]

Pull request overview

This pull request updates the GitHub Actions test workflow to build a local spread binary from a pinned commit instead of relying on a potentially outdated system-provided version. The PR also adds explicit unit test execution and updates the Google Cloud credentials handling to use a file-based approach.

Changes:

• Added a dedicated step to run Go unit tests
• Introduced a step to build a specific version of the spread binary from source
• Modified the spread configuration to use file-based Google Cloud credentials instead of environment variables

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
.github/workflows/test.yaml	Added unit test execution, local spread binary build from pinned commit, and updated credential handling to use key.json file
spread.yaml	Changed Google Cloud key configuration from environment variable to file-based approach

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[niemeyer]

This is curious.. we're moving from keeping the key in memory in an environment variable, to keeping it in the filesystem, but the only place we use it seems to be right next to where we write down. I'm happy to merge this regardless so you can test it, if you wish, but why aren't we just doing what we used to do, which is reading the variable from the environment itself?