extension_ci: Set default target for all jobs (#47712)

Following the changes to the namespace caching, CI in the extensions
broke because cache conflicts arose there seemingly. Trying to set the
target here more explicitly in an effort to fix this.

Release Notes:

- N/A

.github/workflows/extension_tests.yml

@@ -6,6 +6,7 @@ env:
   RUST_BACKTRACE: '1'
   CARGO_INCREMENTAL: '0'
   ZED_EXTENSION_CLI_SHA: 7cfce605704d41ca247e3f84804bf323f6c6caaf
+  CARGO_BUILD_TARGET: wasm32-wasip2
 on:
   workflow_call: {}
 jobs: