chore: specify gmt and gs in ci.yml to try and fix windows testing

Author: mdtanker

.github/workflows/ci.yml

@@ -68,6 +68,9 @@ jobs:
           environment-file: env/testing_env.yml
           create-args: >-
             python=${{ matrix.python-version }}
+            gmt=6.5.0
+            ghostscript=10.04.0
+
           post-cleanup: "all"
           # cache-downloads: false
           # # environment cache is persistent for one week.