Do not mix stable and beta channels

since we have moved to conda-forge, we do not need the stable
channel when building beta packages.

Also, this marks the final change required to conda-forge migration
Fixes #72
6 jobs for separate-channels in 27 minutes and 20 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #247630
docker
build_linux_bob_devel

00:27:20

passed #247632
docker
build_linux_bob_devtools

00:07:52

7.0%
passed #247631
docker
build_linux_deps

00:03:02

passed #247633
intel macos
build_macos_intel_bob_devel

00:25:28

passed #247635
intel macos
build_macos_intel_bob_devtools

00:16:15

7.0%
passed #247634
intel macos
build_macos_intel_deps

00:04:50