default.theme include path resolves to build directory path instead of installed path

# Summary

Since Taskwarrior v3.5.0, running task fails immediately with an error about a missing default.theme file. The path in the error message points to a build-time directory (/build/task/src/task/doc/rc/default.theme) rather than the actual installed location.

# Environment

- Taskwarrior version: 3.5.0
- OS: Arch Linux
- Installation method: pacman package

# Steps to reproduce

1. Install Taskwarrior 3.5.0.
2. Run task.

# Expected behavior

Taskwarrior should start normally and display the task list.

# Actual behavior
```
$ task
Could not read include file '/build/task/src/task/doc/rc/default.theme'.
```
The command exits without any further output.

# Suspected reason
Regression introduced in v3.5.0, most likely by PR #4124.

The path /build/task/src/task/doc/rc/default.theme looks like a build-tree path that was substituted at configure/build time instead of the installation path.

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