Formalize update_engine_version.{sh|ps1}. (#162118)

Towards https://github.com/flutter/flutter/issues/162201.

**NOTE**: This renames the environment variable to
`FLUTTER_PREBUILT_ENGINE_VERSION`.

---

We occasionally break ourselves, our users, and the Dart up (or is it
down? side-ways) stream repos (i.e. HHH) when we change how the
undocumented
[`update_engine_version.sh`](https://github.com/flutter/flutter/blob/master/bin/internal/update_engine_version.sh)
script, and it's Windows counterpart
[`update_engine_version.ps1`](https://github.com/flutter/flutter/blob/master/bin/internal/update_engine_version.ps1)
work, but have no way of knowing until N days/weeks later when someone
tells us.

For example,
<https://flutter-review.googlesource.com/c/recipes/+/62400>.

This is my attempt to encode "this is what you can guarantee by calling
this script".

It _still_ will be an internal only API that we might rev at any time,
but at least we:

1. Can tell the Dart team "this is tested and how it works"
2. If we want to change it, the tests will keep us from changing it
without informing folks it changed

These tests should (in theory) cover both Linux/MacOS and Windows.

/cc @a-siva

.github/workflows/files-changed.yml

@@ -22,7 +22,7 @@ jobs:
         run: |
           git fetch --no-tags --prune --depth=1 origin ${{ github.event.pull_request.base.sha }}
           git fetch --no-tags --prune --depth=1 origin master
-          echo "FLUTTER_ENGINE_VERSION=${{ github.event.pull_request.base.sha }}" >> "$GITHUB_ENV"
+          echo "FLUTTER_PREBUILT_ENGINE_VERSION=${{ github.event.pull_request.base.sha }}" >> "$GITHUB_ENV"
 
       - name: Initialize Dart SDK
         # This downloads the version of the Dart SDK for the current platform.