Starlark flags are omitted from BEP "explicit command line"

### Description of the bug:

Starlark flags (like `label_flag`) are omitted from the `explicitCmdLine` provided via BEP, even when they are in fact explicitly provided by the user on the command line.

This is inconvenient because the user should be able to copy-paste the explicit command line to reproduce the build, but that obviously requires setting the Starlark flags, too!

See https://github.com/buildbuddy-io/buildbuddy/issues/7361 for more context.

### Which category does this issue belong to?

Configurability, Remote Execution

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

_No response_

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

_No response_

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

8.0.0-pre.20240618.2

### 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 HEAD` ?

_No response_

### If this is a regression, please try to identify the Bazel commit where the bug was introduced with bazelisk --bisect.

_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 tracing how Bazel constructs the BEP explicitCmdLine and how Starlark flags such as label_flag are represented when supplied on the command line. The issue provides no reproduction, source file, or test location; done means explicitly supplied Starlark flags appear in explicitCmdLine so the command can reproduce the build.

Written by the indexing model from the issue text.