quote the builtin install instructions

[pmeier]

@dharhas reported offline, that pip install ragna[builtin] doesn't work.

However, pip install ragna[builtin] works. In fact, this is what we use in CI to install our dependencies:

ragna/.github/actions/setup-env/action.yml

Lines 61 to 70 in a04efc2

	- name: Install ragna
	shell: bash -el {0}
	run: |
	if [[ ${{ inputs.optional-dependencies }} == true ]]
	then
	PROJECT_PATH='.[builtin]'
	else
	PROJECT_PATH='.'
	fi
	pip install --editable "${PROJECT_PATH}"

I don't have the actual error in front of me, but I'd wager a guess that something else than bash was used?

In other shells, e.g. zsh, [...] has special meaning, There you would see

❯ pip install ragna[builtin]
zsh: no matches found: ragna[builtin]

Solution to this is simple: quote the offending part:

❯ pip install 'ragna[builtin]'

So this is a "user error". But I can see that this will be common enough. So let's just quote the installation instruction by default, as it has no negative effect other than "looking awkward".