Isolate cargo target dir in auto build tests

[leighmcculloch]

What

Give each auto-build integration test its own CARGO_TARGET_DIR inside its sandbox temp dir.

Why

Parallel tests building the same fixture share one target dir, and the CLI rewrites the output wasm in place after cargo releases its lock, so a concurrent test can read an empty file and fail with unexpected end-of-file (at offset 0x0) (failed run).

Known limitations

Each test now compiles its dependencies from scratch, and concurrent stellar contract build runs on one workspace can still collide for users since the in-place rewrite in build.rs is not atomic.

[Copilot]

🟢 Approval recommended

The targeted test changes consistently isolate build output, with no unresolved findings.

0 open findings

What changed in this PR

This PR isolates Cargo build artifacts for the auto-build integration tests, preventing parallel tests from rewriting the same output file.

Changes:

• Sets CARGO_TARGET_DIR to each test’s sandbox directory for eight deploy and upload auto-build cases.

File	Description
cmd/​crates/​soroban-test/​tests/​it/​integration/​auto_build.rs	Gives each affected test an isolated build target directory.

🧠 Review effort: Lite

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[leighmcculloch]

This change may be unnecessary after this is fixed:

• contract build can expose an empty wasm to concurrent builds #2804

But in general it seems like a better idea to have build tests be isolated, so I think it is still a good idea to merge this. If we find that over time the tests take too long as a result we could wind this back, or look at other solutions.

[leighmcculloch]

Making this change doesn't appear to negatively impact the RPC tests runtime. I guess because they run concurrently. 🎉