Roll buildroot to 7b537de78ac2239982ace130d1845374e5dcf113 (flutter/engine#51053)

Wraps `$python_path` usages in quotes to support python installations in
directories with spaces.

Fixes: https://github.com/flutter/flutter/issues/143740

## Pre-launch Checklist

- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [X] Machine wash cold.
- [X] Do not iron.
- [X] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
instructions on writing and running engine tests.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I signed the [CLA].
- [X] All existing and new tests are passing.

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https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
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[testing the engine]:
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[breaking change policy]:
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DEPS

@@ -279,7 +279,7 @@ allowed_hosts = [
 ]
 
 deps = {
-  'src': 'https://github.com/flutter/buildroot.git' + '@' + '2f684abc6e6bfba9062bd372929a8d458e03940a',
+  'src': 'https://github.com/flutter/buildroot.git' + '@' + '7b537de78ac2239982ace130d1845374e5dcf113',
 
   'src/flutter/third_party/depot_tools':
   Var('chromium_git') + '/chromium/tools/depot_tools.git' + '@' + '580b4ff3f5cd0dcaa2eacda28cefe0f45320e8f7',