APKINDEX.tar.gz: improve repository description

[ollieparanoid]

Right now, the description simply says "user" (because we build the package in /home/user/build).
I have upstreamed a parameter to set any description in abuild. But it will take some time, until it lands in the released version of abuild, that gets actually used in Alpine Linux.

I suggest we wait until they released it, and then simply use the parameter in pmbootstrap to set the description to postmarketOS (maybe with a date appended?). It will probably make sense to add a parameter to pmbootstrap, so we can use another value when building for the binary package repo, than when creating packages locally.