Prevent unintentional Postgres submodule update (#4692)

## Problem

Postgres submodule can be changed unintentionally, and these changes are
easy to miss during the review.

Adding a check that should prevent this from happening, the check fails
`build-neon` job with the following message:
```
Expected postgres-v14 rev to be at '1414141414141414141414141414141414141414', but it is at '1144aee1661c79eec65e784a8dad8bd450d9df79'
Expected postgres-v15 rev to be at '1515151515151515151515151515151515151515', but it is at '1984832c740a7fa0e468bb720f40c525b652835d'
Please update vendors/revisions.json if these changes are intentional.
```
This is an alternative approach to
https://github.com/neondatabase/neon/pull/4603

## Summary of changes
- Add `vendor/revisions.json` file with expected revisions
- Add built-time check (to `build-neon` job) that Postgres submodules
match revisions from `vendor/revisions.json`
- A couple of small improvements for logs from
https://github.com/neondatabase/neon/pull/4603
- Fixed GitHub autocomment for no tests was run case

---------

Co-authored-by: Joonas Koivunen <joonas@neon.tech>

scripts/comment-test-report.js

@@ -205,29 +205,25 @@ module.exports = async ({ github, context, fetch, report }) => {
 
     const {reportUrl, reportJsonUrl} = report
 
-    if (!reportUrl || !reportJsonUrl) {
+    if (reportUrl && reportJsonUrl) {
+        try {
+            const parsed = await parseReportJson({ reportJsonUrl, fetch })
+            commentBody += await reportSummary({ ...parsed, reportUrl })
+        } catch (error) {
+            commentBody += `### [full report](${reportUrl})\n___\n`
+            commentBody += `#### Failed to create a summary for the test run: \n`
+            commentBody += "```\n"
+            commentBody += `${error.stack}\n`
+            commentBody += "```\n"
+            commentBody += "\nTo reproduce and debug the error locally run:\n"
+            commentBody += "```\n"
+            commentBody += `scripts/comment-test-report.js ${reportJsonUrl}`
+            commentBody += "\n```\n"
+        }
+    } else {
         commentBody += `#### No tests were run or test report is not available\n`
-        commentBody += autoupdateNotice
-        return
     }
 
-    try {
-        const parsed = await parseReportJson({ reportJsonUrl, fetch })
-        commentBody += await reportSummary({ ...parsed, reportUrl })
-    } catch (error) {
-        commentBody += `### [full report](${reportUrl})\n___\n`
-        commentBody += `#### Failed to create a summary for the test run: \n`
-        commentBody += "```\n"
-        commentBody += `${error.stack}\n`
-        commentBody += "```\n"
-        commentBody += "\nTo reproduce and debug the error locally run:\n"
-        commentBody += "```\n"
-        commentBody += `scripts/comment-test-report.js ${reportJsonUrl}`
-        commentBody += "\n```\n"
-    }
-
-    commentBody += autoupdateNotice
-
     let createCommentFn, listCommentsFn, updateCommentFn, issueNumberOrSha
     if (isPullRequest) {
         createCommentFn  = github.rest.issues.createComment