Remove redundant builds from unity, godot, csharp, and typescript test suites

[joshua-spacetime]

Description of Changes

Removes redundant CLI and standalone builds from four CI jobs:

1. test
   • Previously cargo ci test built release CLI and standalone.
   • Now uses cargo ci test --prebuilt-runtime.
2. unity-testsuite
   • Previously ran cargo install for CLI and standalone.
   • Now downloads both from build-artifacts-linux.
3. godot-testsuite
   • Previously ran cargo install for CLI and standalone.
   • Now downloads both from build-artifacts-linux.
4. csharp-testsuite
   • Previously ran cargo install for CLI and standalone, while its helper scripts also used Cargo.
   • Now downloads both and supplies the CLI through SPACETIME_BIN.
5. typescript-test
   • Previously built CLI and standalone through Cargo.
   • Now downloads both from build-artifacts-linux and uses cargo ci typescript-test --prebuilt-runtime.
   • Added cargo ci run-spacetime which selects the prebuilt runtime in CI while preserving source builds locally.

API and ABI breaking changes

None

Expected complexity level and risk

2

Testing

The unity, godot, csharp and typescript test suites should be substantially faster which should reduce runner congestion.