CI: Use pixi for all stages of testing (#102)

* Use pixi to provide the runtime environment for removal of the test-package
  from the scientific-python-nightly-wheels channel.
* Use prefix-dev/setup-pixi to setup pixi with no environment created.
   - Use 'run-install: false' to setup pixi with no install.
* Use 'pixi exec --spec' to create a temporary pixi environment with
  anaconda-client installed for the 'anaconda remove' command.

   $ pixi exec --help
   Run a command in a temporary environment

   Usage: pixi exec [OPTIONS] [COMMAND]...

   Arguments:
     [COMMAND]...
             The executable to run

   Options:
     -s, --spec <SPECS>
             Matchspecs of packages to install. If this is not provided, the
             package is guessed from the command

.github/workflows/ci.yml

@@ -96,16 +96,13 @@ jobs:
       name: remove-old-wheels
 
     steps:
-      - name: Install micromamba and anaconda-client
-        uses: mamba-org/setup-micromamba@f8b8a1e23a26f60a44c853292711bacfd3eac822  # v1.9.0
+      - name: Set up pixi
+        uses: prefix-dev/setup-pixi@ba3bb36eb2066252b2363392b7739741bb777659  # v0.8.1
         with:
-          environment-name: remove-wheels
-          create-args: >-
-            python=3.12
-            anaconda-client
+          run-install: false
 
       - name: Remove test package upload
-        shell: bash -l {0}
+        shell: pixi exec --spec anaconda-client -- bash -e {0}
         run: |
             anaconda --token ${{ secrets.ANACONDA_TOKEN }} remove \
               --force \