Add sig files to Github release

Summary:
Currently, our release signatures are available on download.bitcoinabc.org but not on the Github release.
This patch fixes that, making the sigs more easy to find by users.

Test Plan:
github-release.sh -d -a <asset-path> -o <oauth-token-path> -t v0.20.9
Verify the output indicates the sig files from the previous release would be uploaded.

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4818