Remove unnecessary NOT_CRAN flag

Author: thisisnic

dev/tasks/tasks.yml

@@ -674,8 +674,6 @@ tasks:
     template: docker-tests/github.linux.yml
     params:
       image: alpine-linux-r
-      flags: -e NOT_CRAN=false
-
   test-r-macos-as-cran:
     ci: github
     template: r/github.macos.cran.yml