security/keychain: switch to USE_GITHUB
The distfile is no longer found at:
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.
Submitted by: meta (myself)
Reviewed by: email@example.com
Approved by: garga (maintainer)