Use llvm-cov show-env for cts_runner integration tests

[andyleiserson]

May help with #9402, or it may not, I'm not really sure.

The CTS jobs, as the name suggests, run CTS tests. The "other" CTS jobs additionally run the Rust integration tests for cts_runner. Previously, the xtask used llvm-cov show-env so we can run the binary directly instead of via cargo, which makes the runs much faster. But the integration tests were still using cargo llvm-cov run. The show-env method puts build products in their usual locations in target, while llvm-cov run puts them in something like target/llvm-cov-target.

I have two theories of #9402. The one relevant here is that things are generally unhappy switching between the two build directories, so to try and avoid any problems of that sort, this configures the CTS jobs to consistently use show-env.

(The other theory is that when the GitHub actions cache gets saved from a non-"other" job and then used to run the "other", something gets confused about the incremental build state, resulting in the v8 link error. #9403 addressed this by saving caches only from the "other" CTS jobs. (And also only on trunk, but that part is an unrelated optimization.))

Testing
CI, but it's not clear what the precise conditions are for #9402 to show up.

Squash or Rebase? Squash