fix: Better scope pragmas to fix data cube preview query

[bprusinowski]

This PR attempts to better scope pragmas when fetching the cube preview by using a clean Source, without pragmas, as having #pragma join.hash off breaks previews for some cubes for yet unknown reasons (see e.g. https://test.visualize.admin.ch/en/browse?previous=%7B%7D&dataset=https%3A%2F%2Fenvironment.ld.admin.ch%2Ffoen%2Fnfi%2Fnfi_C-94%2Fcube%2F2023-1&dataSource=Prod).

Context

For more information see this conversation on Zulip.

Notes

I aimed for a simple solution by overriding getMainSource View's method, which might not be ideal, but:

• I didn't see a better, simple way of doing so (we can't just pass a clean Source into the View constructor, as this is how it's retrieved internally – taking the source of the first dimension, instead of the one tied to Source),
• this might be a temporary workaround until Stardog releases a new version (https://zulip.zazuko.com/#narrow/stream/40-bafu-ext/topic/performance/near/378807).

How to test

1. Go to the deployment preview.
2. Look at the homepage examples to see if they load fast (<5 s).
3. Go to dataset browse page.
4. Change data source to PROD.
5. Search for a NFI: Topics by ownership cube.
6. Open the preview.
7. See that only 10 observations are loaded.
8. See the same cube preview on TEST and see that 1530 observations are loaded there.

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[ptbrowne]

Thanks, it is somewhat clear to me, and the comment makes it explicit why this was added 👍