install gsl

Author: Tieqiong

Diff for: .github/workflows/tests-on-pr.yml

@@ -35,7 +35,7 @@ jobs:
       - name: Install diffpy.pdfgui and requirements
         run: |
           conda install --file requirements/test.txt
-          conda install wxpython diffpy.utils matplotlib-base
+          conda install wxpython diffpy.utils matplotlib-base gsl
           pip install diffpy.pdffit2==1.4.4rc4
           python -m pip install . --no-deps