install repo2docker into test environment only

No need to install from environment.yml twice, since repo2docker does a separate clean install.

Author: weiji14

.github/workflows/repo2docker.yml

@@ -9,6 +9,9 @@ jobs:
   repo2docker:
     name: Run repo2docker
     runs-on: ubuntu-latest
+    defaults:
+      run:
+        shell: bash -l {0}
 
     steps:
       - name: Checkout
@@ -17,20 +20,15 @@ jobs:
       - name: Setup Miniconda
         uses: conda-incubator/setup-miniconda@835234971496cad1653abb28a638a281cf32541f # v3.2.0
         with:
-          activate-environment: try-gmt
           conda-remove-defaults: "true"
-          environment-file: environment.yml
           miniforge-version: latest
           use-mamba: true
 
-      - name: Install dependencies
-        shell: bash -l {0}
-        run: conda install jupyter-repo2docker
+      - name: Install repo2docker
+        run: mamba install jupyter-repo2docker
 
-      - name: Show installed package
-        shell: bash -l {0}
-        run: conda list
+      - name: Show installed packages
+        run: mamba list
 
       - name: Run repo2docker
-        shell: bash -l {0}
         run: jupyter-repo2docker --no-run --ref ${{ github.event.pull_request.head.sha }} https://github.com/${{ github.event.pull_request.head.repo.full_name }}