s3cmd: add trailing slash to paths for sync from remote

When backing up the contents of object storage, s3cmd sync will download
the contents of the remote bucket every time even if the files are
available locally. Adding the trailing slash allows s3cmd to reuse
existing local files, which is helpful when using persistent storage for
the backup temp space.

This also allows the --delete-removed option to work (which can be
specified in the .s3cfg); otherwise no files are removed. This in turn
is useful when using a regular registry cleanup, which otherwise would
keep growing the local backup copy unbounded.

See also:
- https://github.com/s3tools/s3cmd/issues/245
- https://s3tools.org/s3cmd-sync
24 jobs for s3cmd-sync-fix in 55 minutes and 14 seconds (queued for 24 minutes and 22 seconds)
Status Job ID Name Coverage
  Phase One
passed #255407590
alpine-certificates

00:01:44

passed #255407588
cfssl-self-sign

00:01:45

passed #255407591
gitlab-python

00:07:00

passed #255407585
gitlab-ruby

00:08:12

passed #255407589
kubectl

00:02:06

passed #255407587
postgresql

00:07:22

 
  Phase Two
passed #255407595
gitlab-go

00:02:27

passed #255407597
gitlab-mailroom

00:02:24

passed #255407596
gitlab-monitor

00:02:12

passed #255407593
gitlab-redis-ha

00:03:16

 
  Phase Three
passed #255407599
git-base

00:06:06

passed #255407598
gitlab-shell

00:03:13

 
  Phase Four
passed #255407601
gitaly

00:11:30

passed #255407600
gitlab-elasticsearch-indexer

00:04:17

 
  Phase Five
passed #255407606
gitlab-rails-ce

00:20:20

passed #255407603
gitlab-rails-ee

00:20:44

 
  Phase Six
passed #255407619
gitlab-sidekiq-ce

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passed #255407617
gitlab-sidekiq-ee

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passed #255407611
gitlab-task-runner-ce

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passed #255407607
gitlab-task-runner-ee

00:03:06

passed #255407614
gitlab-unicorn-ce

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passed #255407613
gitlab-unicorn-ee

00:02:50

passed #255407622
gitlab-workhorse-ce

00:04:05

passed #255407620
gitlab-workhorse-ee

00:04:33