Use a simpler workaround for platform mixup issue

[larskanis]

When linux platform comes first, then a rake task is defined for lib/2.7/pg_ext.so which links to linux instead of linux-musl. Defining linux-musl first fixes this since the task is defined only once in: https://github.com/rake-compiler/rake-compiler/blob/ad13dd1ae1000e1ef91b95f3c7782ccca4c1c591/lib/rake/extensiontask.rb#L349

The mixed platform rake definition otherwise leads to a rake dependency chain like this:

rake aborted!
Tasks: TOP => pkg/pg-1.6.0-x86_64-linux-musl.gem => lib/2.7/pg_ext.so => copy:pg_ext:x86_64-linux:2.7.8 => tmp/x86_64-linux/pg_ext/2.7.8/pg_ext.so => tmp/x86_64-linux/pg_ext/2.7.8/Makefile