Update ImportExportUploader cache & work dirs

What does this MR do?

This MR updates cache_dir & work_dir as per request in #205717 (closed) for Project Export to not write anything to gitlab-rails/uploads and write to shared/tmp instead.

Project export was writing to gitlab-rails/uploads due to it inheriting cache_dir and work_dir paths from GitlabUploader https://gitlab.com/gitlab-org/gitlab/blob/master/app/uploaders/gitlab_uploader.rb#L56-62

Dir activity during project export with these changes:

gitlab/shared/tmp/gitlab_exports/@hashed/e0/85/e0850a775c17a87060c0cf6efad1020e0cbef5a44ba942bef6add5776598de53/b0878e051e10e4060be19485028c97bb/f7b6ccf6daf9edc2d5120d20cff5c125/snippets
gitlab/shared/tmp/gitlab_exports/@hashed/e0/85/e0850a775c17a87060c0cf6efad1020e0cbef5a44ba942bef6add5776598de53/b0878e051e10e4060be19485028c97bb/f7b6ccf6daf9edc2d5120d20cff5c125/GITLAB_VERSION
gitlab/shared/tmp/gitlab_exports/@hashed/e0/85/e0850a775c17a87060c0cf6efad1020e0cbef5a44ba942bef6add5776598de53/b0878e051e10e4060be19485028c97bb/f7b6ccf6daf9edc2d5120d20cff5c125/project.json
gitlab/shared/tmp/gitlab_exports/@hashed/e0/85/e0850a775c17a87060c0cf6efad1020e0cbef5a44ba942bef6add5776598de53/b0878e051e10e4060be19485028c97bb/f7b6ccf6daf9edc2d5120d20cff5c125/lfs-objects.json
gitlab/shared/tmp/gitlab_exports/@hashed/e0/85/e0850a775c17a87060c0cf6efad1020e0cbef5a44ba942bef6add5776598de53/b0878e051e10e4060be19485028c97bb/f7b6ccf6daf9edc2d5120d20cff5c125/VERSION
gitlab/shared/tmp/gitlab_exports/@hashed/e0/85/e0850a775c17a87060c0cf6efad1020e0cbef5a44ba942bef6add5776598de53/b0878e051e10e4060be19485028c97bb/f7b6ccf6daf9edc2d5120d20cff5c125/project.bundle
gitlab/shared/tmp/gitlab_exports/@hashed/e0/85/e0850a775c17a87060c0cf6efad1020e0cbef5a44ba942bef6add5776598de53/b0878e051e10e4060be19485028c97bb/f7b6ccf6daf9edc2d5120d20cff5c125/GITLAB_REVISION
gitlab/shared/tmp/gitlab_exports/@hashed/e0/85/e0850a775c17a87060c0cf6efad1020e0cbef5a44ba942bef6add5776598de53/b0878e051e10e4060be19485028c97bb/f7b6ccf6daf9edc2d5120d20cff5c125
gitlab/shared/tmp/work/gitlab_exports/1586198503-44098-0002-9730/2020-04-06_19-41-444_root_asfdasdfsadfsadfsadfsadf_export.tar.gz
gitlab/shared/tmp/work/gitlab_exports/1586198503-44098-0002-9730
gitlab/shared/tmp/cache/gitlab_exports/1586198503-44098-0002-9730/2020-04-06_19-41-444_root_asfdasdfsadfsadfsadfsadf_export.tar.gz
gitlab/public/uploads/-/system/import_export_upload/export_file/235/2020-04-06_19-41-444_root_asfdasdfsadfsadfsadfsadf_export.tar.gz
gitlab/shared/tmp/cache/gitlab_exports/1586198503-44098-0002-9730
gitlab/public/uploads/-/system/import_export_upload/export_file/235/2020-04-06_19-41-637_root_asfdasdfsadfsadfsadfsadf_export.tar.gz
gitlab/shared/tmp/gitlab_exports/@hashed/e0/85/e0850a775c17a87060c0cf6efad1020e0cbef5a44ba942bef6add5776598de53/b0878e051e10e4060be19485028c97bb/2020-04-06_19-41-444_root_asfdasdfsadfsadfsadfsadf_export.tar.gz
gitlab/shared/tmp/gitlab_exports/@hashed/e0/85/e0850a775c17a87060c0cf6efad1020e0cbef5a44ba942bef6add5776598de53/b0878e051e10e4060be19485028c97bb
gitlab/shared/tmp/gitlab_exports/@hashed/e0/85/e0850a775c17a87060c0cf6efad1020e0cbef5a44ba942bef6add5776598de53/locks/0a2835a54a5d792a6e49b495f570c771

There is occurrence of gitlab/public/uploads/-/system since that's what Gitlab.config.uploads['base_dir'] returns and object storage is disabled locally. So that is ok for it to be there. But any mentions of tmp/work and tmp/cache have correct path.

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