force Linux plugin_test to run on Ubuntu 20.04 (#158529)

Work around propagation from https://github.com/flutter/flutter/issues/158521
https://dart.googlesource.com/external/github.com/flutter/flutter/+/8e5bd5af869f11285f9d7fc583c12ee03cf356e3

DEPS

diff --git a/DEPS b/DEPS
index 802d522..d94a140 100644
--- a/DEPS
+++ b/DEPS
@@ -262,7 +262,7 @@
 
   'engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '5cd0517bd66050b4edc7c52fd0f3fa3bd0f37a6d',
 
-  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '9992e9bc5c5c294b9c0f7710c7656bb34d3297c5',
+  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '8e5bd5af869f11285f9d7fc583c12ee03cf356e3',
 
   'engine/src/flutter/third_party/depot_tools':
   Var('chromium_git') + '/chromium/tools/depot_tools.git' + '@' + '580b4ff3f5cd0dcaa2eacda28cefe0f45320e8f7',

commits.json

diff --git a/commits.json b/commits.json
index 1eced53..c10ba54 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "engine/src/flutter":"289a3fb1aa861e37a94101392bd88d1ddb2e88d9",
   "engine/src/flutter/third_party/dart":"5cd0517bd66050b4edc7c52fd0f3fa3bd0f37a6d",
-  "flutter":"9992e9bc5c5c294b9c0f7710c7656bb34d3297c5"
+  "flutter":"8e5bd5af869f11285f9d7fc583c12ee03cf356e3"
 }
\ No newline at end of file