try with bash style string replacement

script/ci/cloudbuild-new-pr-pg11.yaml

@@ -75,7 +75,6 @@ steps:
   args:
     - -cx
     - |
-      _TMP_BRANCH_TAG=$(echo $BRANCH_NAME | sed -r 's,/,_,g')
       docker pull gcr.io/cartodb-on-gcp-main-artifacts/builder:${_BRANCH_TAG}
       if [ ! -z "$?" ]
         then
@@ -94,7 +93,7 @@ steps:
   timeout: 2700s
 
 substitutions: 
-    _BRANCH_TAG: $_TMP_BRANCH_TAG
+    _BRANCH_TAG: ${BRANCH_NAME//\//-}
 options:
     machineType: 'N1_HIGHCPU_32'
 timeout: 3600s

script/ci/cloudbuild-new-pr-pg12.yaml

@@ -75,7 +75,6 @@ steps:
   args:
     - -cx
     - |
-      _TMP_BRANCH_TAG=$(echo $BRANCH_NAME | sed -r 's,/,_,g')
       docker pull gcr.io/cartodb-on-gcp-main-artifacts/builder:${_BRANCH_TAG}
       if [ ! -z "$?" ]
         then
@@ -90,7 +89,6 @@ steps:
   args:
     - -cx
     - |
-      _TMP_BRANCH_TAG=$(echo $BRANCH_NAME | sed -r 's,/,_,g')
       if [ ! -z "${_BRANCH_TAG}" ] && [ ! "${_BRANCH_TAG}" == 'master' ]
       then
         docker push gcr.io/cartodb-on-gcp-main-artifacts/builder:${_BRANCH_TAG}
@@ -108,7 +106,7 @@ steps:
   timeout: 2700s
 
 substitutions: 
-    _BRANCH_TAG: $_TMP_BRANCH_TAG
+    _BRANCH_TAG: ${BRANCH_NAME//\//-}
 options:
     machineType: 'N1_HIGHCPU_32'
 timeout: 3600s