Bazel send host-formatted paths to test-setup and generate-xml scripts

### Description of the bug:

When running tests on a linux remote executor from a Windows host, I see spawns like this:

```
external\bazel_tools\tools\test\test-setup.sh
./cc_test
```
and
```
external\bazel_tools\tools\test\generate-xml.sh
bazel-out/x64_windows-fastbuild/testlogs/cc_test/test.log
bazel-out/x64_windows-fastbuild/testlogs/cc_test/test.xml
0
1
```

Which of course results in `{"msg":"executable file `external\\bazel_tools\\tools\\test\\test-setup.sh` not found in $PATH: No such file or directory","level":"error","time":"2025-12-19T19:26:33.011523Z"}`

The underlying executable is built correctly.
```
/Users/dzbarsky/Downloads/cc_test: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.0.0, not stripped
```

### Which category does this issue belong to?

_No response_

### 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?

Windows

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

release 9.0.0rc2

### 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` ?

```text

```

### 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?

cc @fmeum

Start with external/bazel_tools/tools/test/test-setup.sh and generate-xml.sh, then trace how their paths are constructed for a Linux remote executor from a Windows host. Reproduce the reported spawns if possible and identify the relevant Bazel test setup entry point. Done means both scripts and their test-log paths use valid executor-side formatting, with coverage for this cross-platform case.

Written by the indexing model from the issue text.