chore: Refer release base image on pg branch (#36322)

## Description
As we are referring to the same Postgres version on both the pg and
release branches we can just reuse the same base image for both release
and pg tag. This PR removes the requirement to maintain separate base
tag for the pg branch.

/test Sanity

### 🔍 Cypress test results
<!-- This is an auto-generated comment: Cypress test results  -->
> [!TIP]
> 🟢 🟢 🟢 All cypress tests have passed! 🎉 🎉 🎉
> Workflow run:
<https://github.com/appsmithorg/appsmith/actions/runs/10869515341>
> Commit: 1c7051a
> <a
href="https://internal.appsmith.com/app/cypress-dashboard/rundetails-65890b3c81d7400d08fa9ee5?branch=master&workflowId=10869515341&attempt=1"
target="_blank">Cypress dashboard</a>.
> Tags: `@tag.Sanity`
> Spec:
> <hr>Sun, 15 Sep 2024 08:47:43 UTC
<!-- end of auto-generated comment: Cypress test results  -->


## Communication
Should the DevRel and Marketing teams inform users about this change?
- [ ] Yes
- [ ] No


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Chores**
- Streamlined the logic for determining the base tag in the build
process, simplifying the conditions for setting the `base_tag` variable.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

.github/workflows/build-docker-image.yml

@@ -84,11 +84,7 @@ jobs:
         id: set_base_tag
         run: |
           if [[ "${{ inputs.pr }}" != 0 || "${{ github.ref_name }}" != master ]]; then
-            if [[ ${{ inputs.is-pg-build }} == 'true' || "${{ github.ref_name }}" == pg ]]; then
-              base_tag=pg
-            else
-              base_tag=release
-            fi
+            base_tag=release
           else
             base_tag=nightly
           fi