Add workflow / recipe to generate Dask nightlies (#8469)

As part of addressing dask/community#76, this PR adds:

- A conda recipe in `continuous_integration` to build a nightly `dask-core` package
- A GHA workflow to build this nightly as a check for PRs, and upload this package to the Dask channel under the `dev` label for pushes to `main`

.github/workflows/conda.yml

@@ -0,0 +1,59 @@
+name: Conda build
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+# When this workflow is queued, automatically cancel any previous running
+# or pending jobs from the same branch
+concurrency:
+  group: conda-${{ github.head_ref }}
+  cancel-in-progress: true
+
+# Required shell entrypoint to have properly activated conda environments
+defaults:
+  run:
+    shell: bash -l {0}
+
+jobs:
+  conda:
+    name: Build (and upload)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+      - name: Set up Python
+        uses: conda-incubator/setup-miniconda@v2
+        with:
+          miniforge-variant: Mambaforge
+          use-mamba: true
+          python-version: 3.8
+      - name: Install dependencies
+        run: |
+          mamba install boa conda-verify
+
+          which python
+          pip list
+          mamba list
+      - name: Build conda package
+        run: |
+          # suffix for nightly package versions
+          export VERSION_SUFFIX=a`date +%y%m%d`
+
+          conda mambabuild continuous_integration/recipe \
+                           --no-anaconda-upload \
+                           --output-folder .
+      - name: Upload conda package
+        if: |
+          github.event_name == 'push'
+          && github.ref == 'refs/heads/main'
+          && github.repository == 'dask/dask'
+        env:
+          ANACONDA_API_TOKEN: ${{ secrets.DASK_CONDA_TOKEN }}
+        run: |
+          # install anaconda for upload
+          mamba install anaconda-client
+
+          anaconda upload --label dev noarch/*.tar.bz2

continuous_integration/recipe/meta.yaml

@@ -0,0 +1,47 @@
+{% set version = environ.get('GIT_DESCRIBE_TAG', '0.0.0.dev') + environ.get('VERSION_SUFFIX', '') %}
+{% set py_version=environ.get('CONDA_PY', 36) %}
+
+
+package:
+  name: dask-core
+  version: {{ version }}
+
+source:
+  git_url: ../..
+
+build:
+  number: {{ GIT_DESCRIBE_NUMBER }}
+  noarch: python
+  string: py{{ py_version }}_{{ GIT_DESCRIBE_HASH }}_{{ GIT_DESCRIBE_NUMBER }}
+  script: {{ PYTHON }} -m pip install . -vv
+
+requirements:
+  host:
+    - python >=3.7
+    - pip
+
+  run:
+    - python >=3.7
+    - cloudpickle >=1.1.1
+    - fsspec >=0.6.0
+    - packaging >=20.0
+    - partd >=0.3.10
+    - pyyaml
+    - toolz >=0.8.2
+
+test:
+  imports:
+    - dask
+  commands:
+    - pip check
+  requires:
+    - pip
+
+about:
+  home: https://github.com/dask/dask/
+  summary: Parallel PyData with Task Scheduling
+  license: BSD-3-Clause
+  license_file: LICENSE.txt
+  summary: Parallel Python with task scheduling
+  doc_url: https://dask.org/
+  dev_url: https://github.com/dask/dask