GHA/build_master*: drop branch name from checkout, it is the default (cont.)

Author: vszakats

.github/workflows/build_master.yml

@@ -39,7 +39,6 @@ jobs:
       - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
         with:
           persist-credentials: false
-          ref: 'main'
 
       - name: 'build image: master'
         run: buildah unshare make branch_or_ref=master release_tag=master build_ref_images

.github/workflows/build_master_dev.yml

@@ -39,7 +39,6 @@ jobs:
       - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
         with:
           persist-credentials: false
-          ref: 'main'
 
       - name: 'build image: dev-debian'
         run: buildah unshare make branch_or_ref=master release_tag=master build_debian