[EE] Fix project exports clobbering concurrent export paths

When a project export completes, it removes everything in Project#import_export_shared.archive_path, which can erase files needed for another, ongoing project export. This is problematic for custom templates, which exports an existing project to get the most recent changes and imports that archive to another project.

To avoid this from happening, we generate a random unique subpath in the shared temporary directory so that multiple exports can work at the same time.

Closes #14716 (closed)

Testing Matrix

Test	Export dir deleted?	Locks deleted?	Correct status?	Pass?
Project export	✅	✅	✅	✅
2 simultaneous project exports	✅	✅	✅ The start of the second export invalidates the first, but that happened before	✅
Project export w/ Web upload	✅	✅	✅	✅
1 Project creation w/ custom template	✅	✅	✅	✅
3 simultaneous project creations w/ custom template	✅	✅	✅	✅

Testing methodology

With !16352 (merged), we can now create projects with custom templates via the API, which makes it easy to run tests that require parallelization. For example, to test 3 simultaneous project creations, we can do something like:

curl -v -X POST -H "PRIVATE-TOKEN: ${PRIVATE_TOKEN}" "http://localhost:3001/api/v4/projects?name=parallel1&path=parallel1&use_custom_template=true&template_name=flight" &
curl -v -X POST -H "PRIVATE-TOKEN: ${PRIVATE_TOKEN}" "http://localhost:3001/api/v4/projects?name=parallel2&path=parallel2&use_custom_template=true&template_name=flight" &
curl -v -X POST -H "PRIVATE-TOKEN: ${PRIVATE_TOKEN}" "http://localhost:3001/api/v4/projects?name=parallel3&path=parallel3&use_custom_template=true&template_name=flight" &