fix: respect pre-set environment variables in build template

[Bahtya]

Problem

The generated main.dart in the build template unconditionally overwrites all FLET_* environment variables. This makes it impossible to customize them in containerized or sandboxed environments (e.g., Flatpak on Linux), where the app should respect pre-set system environment variables.

Root Cause

1. environmentVariables is initialized as an empty map {} instead of inheriting from Platform.environment
2. All FLET_* values are set via direct assignment (map["KEY"] = value) which always overwrites

Fix

1. Initialize environmentVariables from Platform.environment so the map starts with all existing system env vars
2. Use putIfAbsent() instead of direct assignment for all FLET_* variables, so pre-set values are preserved while defaults are applied when unset

Before:

Map<String, String> environmentVariables = {};
// ...
environmentVariables["FLET_SERVER_PORT"] = tcpPort.toString(); // always overwrites

After:

Map<String, String> environmentVariables = Map.from(Platform.environment);
// ...
environmentVariables.putIfAbsent("FLET_SERVER_PORT", () => tcpPort.toString()); // respects existing

Fixes #6393

Summary by Sourcery

Preserve and extend environment variables when running the generated Dart main entrypoint so that existing FLET_* settings from the host environment are respected.

Bug Fixes:

• Initialize the Dart build template's environment variable map from the host process environment instead of starting empty, ensuring system-provided variables are retained.
• Apply FLET_* environment variable defaults only when they are not already set, avoiding overwriting pre-configured values in containerized or sandboxed environments.

[CLAassistant]

All committers have signed the CLA.

[sourcery-ai]

We've reviewed this pull request using the Sourcery rules engine

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Updates the Dart build template to preserve host-provided FLET_* environment variables by inheriting the current process environment and only applying defaults when keys are unset (fixes #6393).

Changes:

• Initialize environmentVariables from Platform.environment instead of an empty map.
• Replace direct environmentVariables["KEY"] = ... assignments with putIfAbsent(...) for all FLET_* defaults.

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[ndonkoHenri]

Can you please check this concerning CLA: #6394 (comment) ?

[Bahtya]

Hi @ndonkoHenri, the CLA assistant shows all committers have signed. I've also addressed the Copilot formatting feedback — the indentation issue has been fixed to align with the surrounding block.

[FeodorFitsner]

Change PR base branch to release/v0.85.0 and add CHANGELOG record (there is a skill for that).