Roll buildroot to 6ef931b950d5b9477dba0e278b7006ae327e12f7 (flutter/engine#54384)

Contains:

* [macOS] Fix COPY step on macOS 15 Sequoia (flutter/buildroot#884)
* Bump actions/upload-artifact from 4.3.4 to 4.3.5 (flutter/buildroot#882)
* Bump github/codeql-action from 3.25.14 to 3.25.15 (flutter/buildroot#880)
* Bump ossf/scorecard-action from 2.3.3 to 2.4.0 (flutter/buildroot#881)

Issue: https://github.com/flutter/flutter/issues/152978

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

DEPS

@@ -277,7 +277,7 @@ allowed_hosts = [
 ]
 
 deps = {
-  'src': 'https://github.com/flutter/buildroot.git' + '@' + 'e92499322525306bd637eb79d1e6dd507d5f3062',
+  'src': 'https://github.com/flutter/buildroot.git' + '@' + '6ef931b950d5b9477dba0e278b7006ae327e12f7',
 
   'src/flutter/third_party/depot_tools':
   Var('chromium_git') + '/chromium/tools/depot_tools.git' + '@' + '580b4ff3f5cd0dcaa2eacda28cefe0f45320e8f7',