channels.cfg: switch v20.05 to alpine 3.12

x86_64 packages have been built, let's switch:
http://dl-cdn.alpinelinux.org/alpine/v3.12/

There doesn't seem to be a branch on aports.git yet, but it should
appear soon. So it's master for now.

Closes: #582

channels.cfg

@@ -11,5 +11,6 @@ mirrordir_alpine=edge
 [stable]
 description=Upcoming beta release (WIP, DO NOT USE!)
 branch_pmaports=v20.05
-branch_aports=3.11-stable
-mirrordir_alpine=v3.11
+# will be branched to 3.12-stable
+branch_aports=master
+mirrordir_alpine=v3.12