bazel vendor bazel-external symlink is absolute

### Description of the bug:

In our repo we're trying to vendor our external dependencies so we can ship them in a container image we can use on our CI, we started playing with different settings and found a behavior that's not expected, we can reproduce easily from the Bazel repository

### Which category does this issue belong to?

CLI

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

```bash
$ cd /tmp
$ git clone --depth=1 https://github.com/bazelbuild/bazel
$ cd bazel
$ bazel --output_base=../output_base vendor --vendor_dir=../bazel-vendor src/tools/workspacelog:parser
$ readlink ../bazel-vendor/bazel-external
/tmp/output_base/external
```

This symlink should be relative to be able to move bazel-vendor across multiple environments

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

linux

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

8.3.0

### 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 HEAD` ?

```text
https://github.com/bazelbuild/bazel
71cc37f22d96c27994d66614219e68d44e20023b
```

### 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?

https://bazelbuild.slack.com/archives/C014RARENH0/p1751029538716839

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

_No response_

Reproduce the issue with the provided bazel vendor command and inspect the vendor implementation that creates bazel-external. Verify how the output directory is selected and how the symlink target is written. Done means the vendor directory can move between environments because bazel-external uses a relative target, with a regression test covering this case.

Written by the indexing model from the issue text.