Switch to conda-forge on MacOS (pytorch#13442)

### Summary

This helps mitigate the issue with the broken `llvm-openmp` in trunk

### Test plan
CI

---------

Signed-off-by: Huy Do <huydhn@gmail.com>

Author: huydhn

.ci/scripts/setup-conda.sh

@@ -9,7 +9,7 @@ set -ex
 
 install_conda() {
   pushd .ci/docker || return
-  ${CONDA_INSTALL} -y --file conda-env-ci.txt
+  ${CONDA_INSTALL} -c conda-forge -y --file conda-env-ci.txt
   popd || return
 }