Fix flacks like
https://github.com/zed-industries/zed/actions/runs/33546906778/job/99987145940?pr=63573

`test_extension_store_with_test_extension` gives the extension builder
`target` as its cache at
`crates/extension_host/src/extension_store_test.rs:801`.
`install_dev_extension` performs a real extension and grammar build at
`crates/extension_host/src/extension_host.rs:1139`.
When `target/wasi-sdk` is absent (e.g. after `target/` cleanup), the
builder downloads and extracts it at
`crates/extension/src/extension_builder.rs:511`.

In the failed run, downloading and extracting the SDK consumed 59
seconds, grammar compilation began at 60 seconds, and the timeout fired
immediately.

Release Notes:

- N/A