E2e tests need to ensure that latest stable Dart SDK is in place

[cpswan]

Describe the bug

Dart on the E2e test VM has been manually installed, and occasionally updated. Leading to issues with merging new packages that depend on the latest stable SDK e.g. #1617

The current Dart SDK version is 3.5.0.
Because lints 5.1.0 requires SDK version ^3.6.0-0 and no versions of lints match >5.1.0 <6.0.0, lints ^5.1.0 is forbidden.
So, because sshnoports depends on lints ^5.1.0, version solving failed.
You can try one of the following suggestions to make the pubspec resolve:
* Try using the Dart SDK version: 3.6.0. See https://dart.dev/get-dart.
* Consider downgrading your constraint on lints: dart pub add dev:lints:^5.0.0
2024-12-12 07:57:31.783 | ERROR: Failed to set up binaries - exiting
2024/12/12 07:57:31 Process exited with status 1
Error: Process completed with exit code 1.

Steps to reproduce

1. First there's a new minor release of the Dart
2. Then there are bumps to dependencies that require that release
3. And then e2e tests fail, as the test VM is stuck on the old release

Expected behavior

The e2e test script should ensure that Dart is the latest stable release (preferably using atsign-company/at_dockerfiles/DART_STABLE_VERSION, which we have control over), and update accordingly.

Additional context

This should be incorporated into work for #1444