CI: Install libmamba from conda-forge

Author: oyamad

.github/workflows/ci_np2.yml

@@ -1,9 +1,12 @@
 name: conda-build (NumPy v2)
 
-on: [push]
+on:
+  push:
+  pull_request:
+    branches: [ $default-branch ]
 
 jobs:
-  build-linux:
+  tests:
     runs-on: ubuntu-latest
     strategy:
       max-parallel: 5
@@ -20,6 +23,7 @@ jobs:
         echo $CONDA/bin >> $GITHUB_PATH
     - name: Install dependencies
       run: |
+        conda install -c conda-forge libmamba
         conda env update --file environment_np2.yml --name base
     - name: Conda info
       shell: bash -l {0}