use MKL now that conda-forge supports different BLAS implementations

See https://conda-forge.org/docs/maintainer/knowledge_base.html#switching-blas-implementation
5 jobs for use_mkl in 42 minutes and 6 seconds (queued for 1 second)
Status Job ID Name Coverage
  Mirror
passed #61797
allowed to fail
mirror repository

00:00:26

 
  Build
passed #62060
build image

00:41:40

failed #62059
build image

00:41:09

passed #61821
build image

00:41:34

passed #61798
build image

00:41:20