Add --noarch-build-subdir option

It would be nice to know that we can still build noarch and noarch python on other
OSes (well, except Windows at present). This allows that.

Also, in combination with --output-dir it allows building locally as would happen on
c3i. A sample script to glue this together is:

```
#!/usr/bin/env bash

set -e
FS_STUB=$1

__conda_setup="$(/opt/conda/bin/conda 'shell.bash' 'hook' 2> /dev/null)"
if [ $? -eq 0 ]; then
    eval "$__conda_setup"
fi

if [[ $(uname) == Darwin ]]; then
  BUILD_SUBDIR=osx-64
elif [[ $(uname) =~ M.* ]]; then
  BUILD_SUBDIR=win-64
else
  BUILD_SUBDIR=linux-64
fi

mkdir /tmp/$$.c3i-locally
pushd /tmp/$$.c3i-locally
  conda activate
  cp -rf /opt/r/a/$FS_STUB-feedstock .
  cp -f /opt/r/a/conda_build_config.yaml .
  c3i one-off rdonnelly.$FS_STUB  \
              $FS_STUB-feedstock  \
              --config-root-dir=/opt/asrc/automated-build/c3i_configurations/github_public  \
              --dry-run  \
              --output-dir $PWD  \
              --noarch-build-subdir ${BUILD_SUBDIR}  \
              --no-skip-existing
  mkdir rsync-recipes
  cp -rf *-on-* rsync-recipes/
  mkdir stats
  cmds=$(cat plan.yml | yq -r '.jobs[0].plan[] | select(.task=="build").config.run.args[1]')
  echo $cmds
  conda deactivate
  eval ${cmds}
popd
```

conda_concourse_ci/cli.py

@@ -114,6 +114,10 @@ def parse_args(parse_this=None):
                                 "to all jobs.  For finer control, use extra/worker_tags in "
                                 "meta.yaml with selectors.",
                                 dest='worker_tags')
+    one_off_parser.add_argument('--noarch-build-subdir', '-s', action='store', default='linux-64',
+                                help="set the 'subdir' (machine to build on) for noarch",
+                                dest='buildsubdir')
+
     one_off_parser.add_argument(
         '-m', '--variant-config-files',
         action="append",
@@ -164,7 +168,7 @@ def parse_args(parse_this=None):
         help="""File describing batch job.  Each lines defines a seperate
         one-off job.  List one or more folders on each line.  Job specific
         arguments can be specified after a ';' using param=value, multiple
-        arguments are seperated by a ','.  For example:
+        arguments are separated by a ','.  For example:
 
             recipe-feedstock; channel=conda-forge,clobber_sections_file=clobber.yaml
         """)

conda_concourse_ci/execute.py

@@ -80,7 +80,8 @@ def collect_tasks(
         append_sections_file=None,
         pass_throughs=None,
         skip_existing=True,
-        build_config_vars={}
+        build_config_vars={},
+        build_subdir='linux-64'
         ):
     """ Return a graph of build tasks """
     task_graph = nx.DiGraph()
@@ -126,19 +127,17 @@ def collect_tasks(
         )
         # merge this graph with the main one
         task_graph = nx.compose(task_graph, graph)
-    collapse_noarch_python_nodes(task_graph)
+    collapse_noarch_python_nodes(task_graph, build_subdir=build_subdir)
     return task_graph
 
 
-def collapse_noarch_python_nodes(graph):
+def collapse_noarch_python_nodes(graph, build_subdir='linux-64'):
     """ Collapse nodes for noarch python packages into a single node
 
     Collapse nodes corresponding to any noarch python packages so that each package
     in built on a single platform and test on the remaining platforms.  Edges are
     reassinged or removed as needed.
     """
-    # TODO make build_subdir configurable
-    build_subdir = 'linux-64'
 
     # find all noarch python builds, group by package name
     noarch_groups = defaultdict(list)
@@ -536,6 +535,7 @@ def compute_builds(path, base_name, folders, matrix_base_dir=None,
             raise ValueError(
                     "--destroy-pipeline requires that --push-branch "
                     "and stage-for-upload be specified as well.")
+    build_subdir = kw.get('buildsubdir', 'linux-64')
     folders = folders
     path = path.replace('"', '')
     if not folders:
@@ -591,7 +591,8 @@ def compute_builds(path, base_name, folders, matrix_base_dir=None,
         clobber_sections_file=clobber_sections_file,
         pass_throughs=pass_throughs,
         skip_existing=skip_existing,
-        build_config_vars=build_config_vars
+        build_config_vars=build_config_vars,
+        build_subdir=build_subdir
     )
 
     with open(os.path.join(matrix_base_dir, 'config.yml')) as src: