Reverts "Remove linux_fuchsia builds from release_build: "true". (flutter#168094)" (flutter#168146)

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Reverts: flutter#168094
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Initiated by: matanlurey
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Reason for reverting: I guess it is used.
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Original PR Author: matanlurey
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Reviewed By: {jtmcdole}
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This change reverts the following previous change:
Closes flutter#168089.

These builds are not required in the MQ/part of `beta` or `stable`
branches, and are not requested by the Flutter CLI.

Notably, this deletes the line instead of `release_build: "false"`,
because that is broken:
flutter#168088.
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Co-authored-by: auto-submit[bot] <flutter-engprod-team@google.com>

Author: auto-submit[bot]

engine/src/flutter/.ci.yaml

@@ -144,6 +144,7 @@ targets:
     recipe: engine_v2/engine_v2
     timeout: 120
     properties:
+      release_build: "true"
       config_name: linux_fuchsia
     # Do not remove(https://github.com/flutter/flutter/issues/144644)
     # Scheduler will fail to get the platform