Fix urp udate custom revision jobs

[theopnv]

Purpose of this PR

Copy the unity_revision.txt file inside the URPUpdate folder, where it can be picked up by unity-downloader-cli.
This only affects CUSTOM-REVISION tests.

---

Testing status

Without this PR: The job is stuck because it can't find the unity_revision.txt file: https://yamato.cds.internal.unity3d.com/jobs/902-Graphics/tree/master/.yamato%252Furpupdate_top_asset_store-win-dx11.yml%2523URPUpdate_Top_Asset_Store_Win_DX11_playmode_mono_Linear_CUSTOM-REVISION/4885113/job/(log:Execution)

With this PR: https://yamato.cds.internal.unity3d.com/jobs/902-Graphics/tree/yamato%252Ffix-urpudate-tests/.yamato%252Furpupdate_top_asset_store-win-dx11.yml%2523URPUpdate_Top_Asset_Store_Win_DX11_playmode_mono_Linear_CUSTOM-REVISION/4887085/job

The tests are failing but the failure is also happening on master.

[theopnv]

To backport this on 10.x.x:
Depends on #3170 and #3175 to be backported first.

To backport this on 2021.1:
Depends on #3175

[theopnv]

Note about the backports:
I think on master this PR was reverted inadvertently since moving the yml files generator to the set repo because I can't find the piece of code it added https://github.com/Unity-Technologies/Graphics/blob/master/.yamato/urpupdate-win-dx11.yml#L89.
I'm running https://yamato.cds.internal.unity3d.com/jobs/902-Graphics/tree/master/.yamato%252Furpupdate_top_asset_store-win-dx11.yml%2523URPUpdate_Top_Asset_Store_Win_DX11_playmode_mono_Linear_CUSTOM-REVISION/5570699/job on master to see if it's broken or not and will file a bug report if it is.
In that case, and since the code has changed I will remove the needs-10.xbackport label, to create the PR from scratch directly on 10.x.x.

EDIT: The job has passed so I believe the code from this PR was correctly incorporated into the yaml generator refactor. There is no need to backport anything since the python code is now the same for all branches.