fix(config-dir): resolve relative TURBO_CONFIG_DIR_PATH before validation

Fixes issue #8645

Summary

This PR fixes a bug where setting TURBO_CONFIG_DIR_PATH to a relative path
(e.g., "configs") caused Turborepo to fail with:

PathError::NotAbsolute("configs")

AbsoluteSystemPathBuf::new() only accepts absolute paths, but the value from
the environment variable was passed directly without being resolved.
This caused a startup failure even though the provided path was valid.

Root Cause

When a user sets: TURBO_CONFIG_DIR_PATH="configs"

Turborepo attempted to validate "configs" as an absolute path.
Since it is relative, the validator rejected it, causing Turborepo to stop early.

What This PR Changes

• Detects whether the provided TURBO_CONFIG_DIR_PATH is relative

• If relative, resolves it against the current working directory:

  std::env::current_dir()?.join(raw)

Converts the resolved path to UTF-8
Passes the resulting absolute path into AbsoluteSystemPathBuf::new()
Leaves absolute paths fully unchanged
Introduces zero breaking changes

Why This Fix Is Safe

• Relative → absolute resolution is deterministic and standard practice
• Absolute paths behave exactly the same as before
• Error surface area decreases (fewer startup failures)
• The change is minimal, well-scoped, and consistent with existing path logic
• No impact on users who do not set the environment variable