CI: temporary disable coverage report for regression tests (#6798)

## Problem

The merging coverage data step recently started to be too flaky.
This failure blocks staging deployment and along with the flakiness of
regression tests might require 4-5-6 manual restarts of a CI job.

Refs:
- https://github.com/neondatabase/neon/issues/4540
- https://github.com/neondatabase/neon/issues/6485
- https://neondb.slack.com/archives/C059ZC138NR/p1704131143740669

## Summary of changes
- Disable code coverage report for functional tests

.github/workflows/build_and_test.yml

@@ -473,8 +473,12 @@ jobs:
           BUILD_TAG: ${{ needs.tag.outputs.build-tag }}
           PAGESERVER_VIRTUAL_FILE_IO_ENGINE: std-fs
 
+      # Temporary disable this step until we figure out why it's so flaky
+      # Ref https://github.com/neondatabase/neon/issues/4540
       - name: Merge and upload coverage data
-        if: matrix.build_type == 'debug' && matrix.pg_version == 'v14'
+        if: |
+          false &&
+          matrix.build_type == 'debug' && matrix.pg_version == 'v14'
         uses: ./.github/actions/save-coverage-data
 
   get-benchmarks-durations:

scripts/comment-test-report.js

@@ -188,7 +188,7 @@ const reportSummary = async (params) => {
 }
 
 const parseCoverageSummary = async ({ summaryJsonUrl, coverageUrl, fetch }) => {
-    let summary = `\n### Code coverage ([full report](${coverageUrl}))\n`
+    let summary = `\n### Code coverage* ([full report](${coverageUrl}))\n`
 
     const coverage = await (await fetch(summaryJsonUrl)).json()
     for (const covType of Object.keys(coverage).sort()) {
@@ -198,7 +198,7 @@ const parseCoverageSummary = async ({ summaryJsonUrl, coverageUrl, fetch }) => {
 
         summary += `- \`${covType}s\`: \`${coverage[covType]["_summary"]}\`\n`
     }
-
+    summary += "\n\\* collected from Rust tests only\n"
     summary += `\n___\n`
 
     return summary