ci: fix conda config for platform tests

[skip conda][skip rhel][skip debian][skip wheels][skip docs][skip compiler][skip lint][skip integration]
6 jobs for ci-rewrite in 49 minutes and 18 seconds (queued for 1 second)
Status Job ID Name Coverage
  Tarballs
passed #1212743
tarball:lal

00:01:29

passed #1212742
tarballs

00:06:03

 
  Platform Tests
passed #1212745
platform:conda:linux

00:32:34

passed #1212746
macos
platform:conda:macos

00:49:18

passed #1212744
platform:debian:stretch

00:41:45

 
  Coverage
passed #1212747
coverage

00:48:18