test: add Go CLI test harness; remove superseded integration targets

[gilescope]

less to go wrong, faster

[github-actions]

⚠️ Are we earthbuild yet?

Warning: "earthly" occurrences have increased by 21 (0.4%)

📈 Overall Progress

Branch	Total Count
main	5315
This PR	5336
Difference	+21 (0.4%)

📁 Changes by file type:

File Type	Change
Go files (.go)	❌ +17
Documentation (.md)	➖ No change
Earthfiles	❌ +4

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Keep up the great work migrating from Earthly to Earthbuild! 🚀

💡 Tips for finding more occurrences

Run locally to see detailed breakdown:

./.github/scripts/count-earthly.sh

Note that the goal is not to reach 0.
There is anticipated to be at least some occurences of earthly in the source code due to backwards compatibility with config files and language constructs.

[gemini-code-assist]

Code Review

This pull request migrates several Earthfile-based integration tests (such as builtin-args, let-scope, let-set, infinite-recursion, and cache-cmd) into a new Go-based CLI test suite in tests/cli/cli_test.go, accompanied by corresponding test fixtures. The review feedback suggests using a stable Go version (like 1.23 instead of 1.25) in the generated go.mod fixture to prevent build failures, and recommends avoiding if statements with initializers to comply with potential linting rules.

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