fix: do not change cwd if pointing to _quarto.yml

[machow]

When pointing to a quarto file (e.g. quartodoc build --config docs/_quarto.yml), we no longer change the cwd, but append the docs folder holding _quarto.yml (e.g. docs/) to the path. This

• continues to support cases where people define a renderer in their quarto docs folder, but call it from home.
• fixes an issue where building a subfolder changed your working directory, which you relied on for importing your package.

It also just feels a bit less spooky.