Need a way to generate .exec file instead of .dat file for coverage

### Description of the feature request:

When there is a unit test run completion, bazel generate coverage.dat file for coverage.
We have a custom use case where we need .exec file.
We need a way to control this.

### Which category does this issue belong to?

Core

### What underlying problem are you trying to solve with this feature?

_No response_

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

_No response_

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

_No response_

### 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_

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

_No response_

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

_No response_

No file, test, or entry point is named. Begin by locating Bazel's coverage output handling and the code that produces coverage.dat, then determine how output format selection should work and what coverage tests would verify the requested .exec behavior.

Written by the indexing model from the issue text.