Don't run caching steps on CI when not master or PR

.drone.yml

@@ -12,6 +12,9 @@ steps:
       restore: true
       mount:
       - vendor
+    when:
+      branch:
+        - master
   - name: rspec
     image: guildeducation/rails:2.7.1-12.19.0
     commands:
@@ -30,6 +33,9 @@ steps:
       rebuild: true
       mount:
       - vendor
+    when:
+      branch:
+        - master
 
 volumes:
   - name: cache