security/keychain: switch to USE_GITHUB

The distfile is no longer found at:
http://www.funtoo.org/archive/keychain/keychain-2.8.5.tar.bz2

As mentioned in WWW, the distfile can be found on GitHub.

The latest release of Keychain can be found on the keychain GitHub release page.

While here, switch WWW to https and changed to nice-look URL.

PR: 245808
Submitted by: meta (myself)
Reviewed by: daniel.engberg.lists@pyret.net
Approved by: garga (maintainer)