dev: Switch away from the now-deprecated provision-with-micromamba

It was deprecated last week in favor of a new setup-micromamba action¹,
but that doesn't put `micromamba` on PATH without extra faffing about.

Since we use the action merely to obtain a `micromamba` executable on the
PATH—shell init, environment creation, etc. is all unnecessary for our
purposes—switch to the simpler installer shell script.²

¹ <https://github.com/mamba-org/provision-with-micromamba/releases/tag/v16>
² <https://mamba.readthedocs.io/en/latest/installation.html#install-script>

.github/workflows/ci.yaml

@@ -38,9 +38,10 @@ jobs:
     name: build and test (os=${{ matrix.os }})
     runs-on: ${{ matrix.os }}
     steps:
-      - uses: mamba-org/provision-with-micromamba@v15
-        with:
-          environment-file: false
+      - name: Install Micromamba
+        run: |
+          curl -fsSL --proto 'https=' https://micro.mamba.pm/install.sh | bash
+          echo ~/.local/bin >> "$GITHUB_PATH"
 
       - uses: actions/checkout@v3
 
@@ -87,9 +88,10 @@ jobs:
       - build-and-test
     runs-on: ubuntu-22.04
     steps:
-      - uses: mamba-org/provision-with-micromamba@v15
-        with:
-          environment-file: false
+      - name: Install Micromamba
+        run: |
+          curl -fsSL --proto 'https=' https://micro.mamba.pm/install.sh | bash
+          echo ~/.local/bin >> "$GITHUB_PATH"
 
       - uses: actions/checkout@v3
       - uses: actions/download-artifact@v3