Use string separator in nightly version string (#295)

Author: charlesbluca

.github/workflows/test.yml

@@ -216,7 +216,7 @@ jobs:
         shell: bash -l {0}
         run: |
           # suffix for nightly package versions
-          export VERSION_SUFFIX=`date +%y%m%d`
+          export VERSION_SUFFIX=a`date +%y%m%d`
 
           mamba build continuous_integration/recipe \
                       --no-anaconda-upload \