Followup fixes to manifests.txt

[emilyalbini]

This PR contains multiple fixes to #89:

• It now uses --if-match instead of --if-none-match when the file exists already, as noted in Update manifests.txt from the release process #89 (comment)
• It adds new manifests at the bottom of the file rather than at the top
• It handles releases with multiple manifests (for example, one for beta and one for 1.89-beta in the case of beta releases)

I managed to test them locally and everything seems to work fine.