Remove github CI 'build_postgres' job, merging it with 'build_neon'

Simplifies the workflow. Makes the overall build a little faster, as
the build_postgres step doesn't need to upload the pg.tgz artifact,
and the build_neon step doesn't need to download it again.

This effectively reverts commit a490f64a68. That commit changed the
workflow so that the Postgres binaries were not included in the
neon.tgz artifact. With this commit, the pg.tgz artifact is gone, and
the Postgres binaries are part of neon.tgz again.

.github/actions/run-python-test-set/action.yml

@@ -37,12 +37,6 @@ runs:
         name: neon-${{ runner.os }}-${{ inputs.build_type }}-${{ inputs.rust_toolchain }}-artifact
         path: ./neon-artifact/
 
-    - name: Get Postgres artifact for restoration
-      uses: actions/download-artifact@v3
-      with:
-        name: postgres-${{ runner.os }}-${{ inputs.build_type }}-artifact
-        path: ./pg-artifact/
-
     - name: Extract Neon artifact
       shell: bash -ex {0}
       run: |
@@ -50,13 +44,6 @@ runs:
         tar -xf ./neon-artifact/neon.tgz -C /tmp/neon/
         rm -rf ./neon-artifact/
 
-    - name: Extract Postgres artifact
-      shell: bash -ex {0}
-      run: |
-        mkdir -p /tmp/neon/tmp_install
-        tar -xf ./pg-artifact/pg.tgz -C /tmp/neon/tmp_install
-        rm -rf ./pg-artifact/
-
     - name: Checkout
       if: inputs.needs_postgres_source == 'true'
       uses: actions/checkout@v3
@@ -78,7 +65,7 @@ runs:
     - name: Run pytest
       env:
         NEON_BIN: /tmp/neon/bin
-        POSTGRES_DISTRIB_DIR: /tmp/neon/tmp_install
+        POSTGRES_DISTRIB_DIR: /tmp/neon/pg_install
         TEST_OUTPUT: /tmp/test_output
         # this variable will be embedded in perf test report
         # and is needed to distinguish different environments