CI: use build-tools image from dockerhub (#6795)

## Problem

Currently, after updating `Dockerfile.build-tools` in a PR, it requires
a manual action to make it `pinned`, i.e., the default for everyone. It
also makes all opened PRs use such images (even created in the PR and
without such changes).
This PR overhauls the way we build and use `build-tools` image (and uses
the image from Docker Hub).

## Summary of changes
- The `neondatabase/build-tools` image gets tagged with the latest
commit sha for the `Dockerfile.build-tools` file
- Each PR calculates the tag for `neondatabase/build-tools`, tries to
pull it, and rebuilds the image with such tag if it doesn't exist.
- Use `neondatabase/build-tools` as a default image
- When running on `main` branch — create a `pinned` tag and push it to
ECR
- Use `concurrency` to ensure we don't build `build-tools` image for the
same commit in parallel from different PRs

CONTRIBUTING.md

@@ -74,16 +74,11 @@ We're using the following approach to make it work:
 
 For details see [`approved-for-ci-run.yml`](.github/workflows/approved-for-ci-run.yml)
 
-## How do I add the "pinned" tag to an buildtools image?
-We use the `pinned` tag for `Dockerfile.buildtools` build images in our CI/CD setup, currently adding the `pinned` tag is a manual operation.
+## How do I make build-tools image "pinned"
 
-You can call it from GitHub UI: https://github.com/neondatabase/neon/actions/workflows/update_build_tools_image.yml,
-or using GitHub CLI:
+It's possible to update the `pinned` tag of the `build-tools` image using the `pin-build-tools-image.yml` workflow.
 
 ```bash
-gh workflow -R neondatabase/neon run update_build_tools_image.yml \
-            -f from-tag=6254913013 \
-            -f to-tag=pinned \
-
-# Default `-f to-tag` is `pinned`, so the parameter can be omitted.
-```
+gh workflow -R neondatabase/neon run pin-build-tools-image.yml \
+            -f from-tag=cc98d9b00d670f182c507ae3783342bd7e64c31e
+```