[URP][case 1348209] fix for gray thumbnails

[martint-unity]

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Purpose of this PR

Fixing gray thumbnails issue here https://fogbugz.unity3d.com/f/cases/1348209/

Before: The thumbnails didn't really render correctly and were only gray squares.

After:

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Testing status

Opened a new template scene and tested it.

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Comments to reviewers

Notes for the reviewers you have assigned.

[github-actions]

Hi! This comment will help you figure out which jobs to run before merging your PR. The suggestions are dynamic based on what files you have changed.
Link to Yamato: https://yamato.cds.internal.unity3d.com/jobs/902-Graphics
Search for your PR branch using the sidebar on the left, then add the following segment(s) to the end of the URL (you may need multiple tabs depending on how many packages you change)

URP
/.yamato%252Fall-urp.yml%2523PR_URP_trunk
With changes to URP packages, you should also run
/.yamato%252Fall-lightmapper.yml%2523PR_LightMapper_trunk

Depending on the scope of your PR, you may need to run more jobs than what has been suggested. Please speak to your lead or a Graphics SDET (#devs-graphics-automation) if you are unsure.

[ernestasKupciunas]

Looks good! Thanks for clear description and images!

In addition, I have also manually tested the URP template and upgradability from built-in to URP. Thumbnails are visible.

[martint-unity]

The tests that were failing also fails on Nightly tests so shouldn't be anything this PR introduces.