CI: Remove old scipy-wheels-nightly uploads to ensure space

Remove all but the last "${N_LATEST_UPLOADS}" package version uploads
to the matplotlib scipy-wheels-nightly index to ensure space. To do this,
rely on the output form of `anaconda show` to be able to filter on the item
delimiter character sequence for each version currently uploaded.

As an explicit example:

```
$ anaconda show scipy-wheels-nightly/matplotlib
Using Anaconda API: https://api.anaconda.org
Name:    matplotlib
Summary:
Access:  public
Package Types:  pypi
Versions:
   + 3.6.0.dev2553+g3245d395d9
   + 3.6.0.dev2569+g3522217386
   + 3.6.0.dev2573+g3eadeacc06

To install this package with pypi run:
     pip install -i https://pypi.anaconda.org/scipy-wheels-nightly/simple matplotlib
```

shows that by filtering on '+' and then stripping ' + ' one can obtain a
newline separated list of all package uploads, where they _most recent_
uploads are listed last. After stripping off the "${N_LATEST_UPLOADS}" lines
that correspond to the package versions to keep for testing, the remaining (older)
package uploads can be removed with `anaconda remove`.

Resolves the space problem in https://github.com/matplotlib/matplotlib Issue 22757

Author: matthewfeickert

.github/workflows/nightlies.yml

@@ -63,3 +63,23 @@ jobs:
             --user scipy-wheels-nightly \
             --skip-existing \
             dist/matplotlib-*.whl
+
+      - name: Remove old uploads to save space
+        shell: bash
+        run: |
+          N_LATEST_UPLOADS=5
+
+          # Remove all _but_ the last "${N_LATEST_UPLOADS}" package versions
+          # N.B.: `anaconda show` places the newest packages at the bottom of the output
+          # of the 'Versions' section and package versions are preceded with a '   + '.
+          anaconda show scipy-wheels-nightly/matplotlib &> >(grep '+') | \
+              sed 's/.* + //' | \
+              head --lines "-${N_LATEST_UPLOADS}" > remove-package-versions.txt
+
+          if [ -s remove-package-versions.txt ]; then
+              while LANG=C IFS= read -r package_version ; do
+                  anaconda --token ${{ secrets.ANACONDA_ORG_UPLOAD_TOKEN }} remove \
+                    --force \
+                    "scipy-wheels-nightly/matplotlib/${package_version}"
+              done <remove-package-versions.txt
+          fi