[full-ci] [tests-only] Fix su exec ci

[phil-davis]

Description

1. su-exec is no longer provided in the owncloud-ci docker images, so do not use it, use setpriv (similar to refactor: replace su-exec/gosu with setpriv owncloud-docker/base#231 )
2. setpriv is only in Ubuntu 20.04, so use that for running tests. (cherry-picked from [tests-only] Run PHP tests with 7.4-ubuntu20.04 #40385 - we were thinking to do this anyway, and now we have to)
3. adjust the construction of pipeline names to avoid some names getting longer than 50 characters

This is needed to get CI green again. CI broke when owncloudci/php Docker was rebuilt recently, and base images no longer had su-exec in them.

How Has This Been Tested?

CI

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Database schema changes (next release will require increase of minor version instead of patch)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:
• Changelog item, see TEMPLATE

[phil-davis]

Note: this PR explicitly uses the owncloudci/php:7.4-ubuntu20.04 docker image.

owncloudci/php:7.4 uses Ubuntu 18.04 - next week I will think about if we should change that so it uses Ubuntu 20.04. But for now, we want to get CI green.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

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