Coverage reports are not built reliably

### Description of the bug:

https://github.com/bazelbuild/rules_rust/issues/2079 shows coverage reports, despite using [--experimental_fetch_all_coverage_outputs](https://bazel.build/reference/command-line-reference#flag--experimental_fetch_all_coverage_outputs), are not consistently built.

### What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

I unfortunately don't have a reliable repro, the main pipeline (post-merge) regularly fails though. My thoughts are it happens more to builds that have full cache hits.

### Which operating system are you running Bazel on?

Linux, MacOS

### What is the output of `bazel info release`?

6.3.0

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse master; git rev-parse HEAD` ?

_No response_

### Is this a regression? If yes, please try to identify the Bazel commit where the bug was introduced.

_No response_

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

_No response_

Start by examining the post-merge coverage pipeline on Linux and macOS with --experimental_fetch_all_coverage_outputs enabled, comparing builds with full cache hits. Use the linked rules_rust issue as context; done means coverage reports are built consistently and the failure has a reliable reproduction.

Written by the indexing model from the issue text.