fix: set QT scale env before launching dde-update in X11

[MyLeeJiEun]

1. Fix AfterLogin progress ball and welcome text displayed too small after system upgrade due to inconsistent scaling
2. Check xsettingsd.conf existence before launching dde-update
3. Run xsettingsd with config if xsettingsd.conf exists
4. Fall back to greeter-display-setting for QT scale env vars

Log: Fix AfterLogin UI elements too small after upgrade by setting QT scale env in X11

fix: 在 X11 下启动 dde-update 前设置 QT 缩放环境变量

1. 修复系统升级后 AfterLogin 进度球和欢迎界面文案偏小、缩放与系统不一致的问题
2. 在启动 dde-update 前检查 xsettingsd.conf 是否存在
3. 若存在则使用该配置启动 xsettingsd
4. 若不存在则通过 greeter-display-setting 获取并导出 QT 缩放环境变量

Log: 修复系统升级后 AfterLogin 界面元素偏小的问题，在 X11 下提前设置 QT 缩放环境变量
PMS: BUG-363285

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[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: MyLeeJiEun, yixinshark

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[MyLeeJiEun]

/merge