Avoid quotes in suggested .env file for IDEs

[huonw]

This fixes #20127 by avoiding double quotes in the .env file generated by the code snippet in the IDE set-up docs.

Double quotes aren't supported by the scie-pants .env loader, and a line containing them was previously silently ignored (plus all lines after), but with scie-pants 0.11.0, will be an error. See pantsbuild/scie-pants#307 and pantsbuild/scie-pants#319.

The new suggestion has no quotes at all, because there's no other way to have a line that includes a substitution. To handle source roots with spaces in their paths, this has to also manually escape them. Supporting double quotes is effectively a new feature of scie-pants launcher (via scie jump a-scie/jump#166 via dotenvy allan2/dotenvy#11), but for now we can at least stop suggesting incorrect things.

For instance, with source roots a b/c and def, this generates:

PYTHONPATH=./a\ b/c:./def:$PYTHONPATH

[WorkerPants]

I tried to automatically cherry-pick this change back to each relevant milestone, so that it is available in those older releases of Pants.

✔️ 2.17.x

Successfully opened #20145.

❌ 2.18.x

I was unable to cherry-pick this PR to 2.18.x, likely due to merge-conflicts.

Steps to Cherry-Pick locally

To resolve:

1. (Ensure your git working directory is clean)
2. Run the following script to reproduce the merge-conflicts:

   git fetch https://github.com/pantsbuild/pants main \
     && git fetch https://github.com/pantsbuild/pants 2.18.x \
     && git checkout -b cherry-pick-20144-to-2.18.x FETCH_HEAD \
     && git cherry-pick e91d6e7d7d830815424b64332b857a22729dd5b6

3. Fix the merge conflicts and commit the changes
4. Run build-support/cherry_pick/make_pr.sh "20144" "2.18.x"

Please note that I cannot re-run CI if a job fails. Please work with your PR approver(s) to re-run CI if necessary.

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When you're done manually cherry-picking, please remove the needs-cherrypick label on this PR.

Thanks again for your contributions!

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