Skip existing release artifacts when retrying mirror uploads (#4937)

A mirror upload can fail after some files for a release have already
reached R2. Retrying then attempts to overwrite those immutable objects
and can fail again.

This change makes the versioned mirror upload skip existing keys and
upload only the missing files. It assumes a retry uses the same release
artifacts. An existing file is kept even if its local checksum differs.

Tested against a local S3 server with a partially uploaded release. The
first run preserved the existing file and uploaded the missing files. A
second run made zero writes.

Author: ewdurbin

.github/workflows/publish-mirror.yml

@@ -48,7 +48,7 @@ jobs:
           R2_BUCKET: ${{ secrets.MIRROR_R2_BUCKET_NAME }}
           PROJECT: ty
         run: |
-          aws s3 cp --recursive --output table --color on \
+          aws s3 sync --no-overwrite --output table --color on \
             --exclude '*' \
             --include '*.zip' --include '*.zip.sha256' \
             --include '*.tar.gz' --include '*.tar.gz.sha256' \