ci: ignore prereleases when publishing manifest

[strazto]

Early releases gave invalid version strings:

jellyfin-plugin-sso/manifest.json

Lines 59 to 72 in b8439bc

	"changelog": "Add Google support, code cleanup.\r\n\r\nAdmin page still isn't working",
	"checksum": "",
	"sourceUrl": "https://github.com/9p4/jellyfin-plugin-sso/releases/download/v2.0.0.0-alpha/sso-authentication_2.0.0.0.zip",
	"targetAbi": "10.8.0.0",
	"timestamp": "2022-01-23T21:54:18Z",
	"version": "1"
	},
	{
	"changelog": "This is the initial alpha release! Feedback and PRs welcome.\r\n\r\nSupports Jellyfin 10.8.",
	"checksum": "",
	"sourceUrl": "https://github.com/9p4/jellyfin-plugin-sso/releases/download/v1.0.0.0-alpha/sso-authentication_1.0.0.0.zip",
	"targetAbi": "10.8.0.0",
	"timestamp": "2022-01-17T03:40:12Z",
	"version": "1"

I've marked these as prerelease & valid installation candidates as releases, and instructed jprm action to ignore them