dev: Upgrade setup-miniconda action from v2 → v3

The latest version will now use Conda's osx-arm64 channel on macOS when
available.  While the original reason for this change was because we
were encountering issues with osx-64 packages on arm64 hardware¹, this
upgrade alone wasn't enough to fix our issues.²  Still, let's upgrade
since v3 is the supported version, and it'll be good to use non-emulated
osx-arm64 binaries going forward anyway.

¹ <#382>
² <#384 (comment)>

.github/actions/setup-integration-tests/action.yaml

@@ -22,7 +22,7 @@ inputs:
 runs:
   using: composite
   steps:
-    - uses: conda-incubator/setup-miniconda@v2
+    - uses: conda-incubator/setup-miniconda@v3
       with:
         python-version: ${{ inputs.python-version }}
         miniforge-variant: Mambaforge