fix(config): resolve roots relative to explicit config paths

[cayasde]

Reproduction

Create this layout:

games/arena/lutest.toml
games/arena/src/inventory.luau

games/arena/lutest.toml:

[discovery.lute]
require = "@lutest_release"
roots = ["src"]

From the repository root, run:

lutest test --config games/arena/lutest.toml --dry-run

Actual behavior

Lutest resolves roots = ["src"] from the current directory and fails:

x Configured discovery root 'src' does not exist.

> <repository-root>/lutest.toml

The Where path is also misleading: the selected config was games/arena/lutest.toml.

Running the same command after changing into games/arena succeeds.

Expected behavior

When --config <path> is used, relative configuration values such as discovery.*.roots should resolve from the configuration file's directory. This makes a nested config portable and matches the monorepo use case for the option.