Stop setting GIT_BASE_SHA in workflows, only set it in makefiles to make it work for push not only for PR.

Author: dysk

.github/workflows/pricing-mutate.yml

@@ -31,5 +31,3 @@ jobs:
           working-directory: ecommerce/pricing
       - name: Run incremental mutation testing
         run: make -C ecommerce/pricing mutate-changes
-        env:
-          GIT_BASE_SHA: ${{ github.event.pull_request.base.sha }}

.github/workflows/rails_application-mutate.yml

@@ -55,4 +55,3 @@ jobs:
         env:
           DATABASE_URL: "postgres://postgres:secret@localhost:5432/cqrs-es-sample-with-res_test"
           RAILS_ENV: test
-          GIT_BASE_SHA: ${{ github.event.pull_request.base.sha }}