github_packages: update mediaType.

[MikeMcQuaid]

This custom type will allow better display in GitHub Packages.

[BrewTestBot]

Review period skipped due to critical label.

[sjackman]

Rather than change the mediaType, I'd prefer to use an annotation indicating that it's a Homebrew bottle. Changing the mediaType risks that third-party tools that would otherwise work with our OCI could fail if they validate the mediaType.

Further conversation with GitHub here has indicated why they can't use an annotation for this.

[sjackman]

After merging this PR, we are unable to use skopeo to copy the resulting OCI image.

$ brew pr-upload --no-commit --github-org=brewsci
…
Error: Failure while executing; `/usr/local/bin/skopeo copy --all oci:seqkit--0.15.0 docker://ghcr.io/brewsci/bio/seqkit:0.15.0` exited with 1. Here's the output:
Getting image list signatures
Copying 2 of 2 images in list
Copying image sha256:68fa14405fdf83b46275c18c66705a55456b8ba4294a97367164f9182000d3de (1/2)
time="2021-04-01T16:10:37-07:00" level=fatal msg="Error initializing image from source oci:seqkit--0.15.0:: unsupported docker v2s2 media type: \"\""
$ skopeo copy --all oci:seqkit--0.15.0 oci:seqkit--0.15.0.copy
Getting image list signatures
Copying 2 of 2 images in list
Copying image sha256:68fa14405fdf83b46275c18c66705a55456b8ba4294a97367164f9182000d3de (1/2)
FATA[0000] Error initializing image from source oci:seqkit--0.15.0:: unsupported docker v2s2 media type: ""