fix: must set CMAKE_INSTALL_PREFIX before GNUInstallDirs

[wineee]

Log: Never modify the value of CMAKE_INSTALL_PREFIX after including GNUInstallDirs ，Otherwise incorrect CMAKE_INSTALL_FULL_XXXX values will be computed

[deepin-bot[bot]]

Doc Check bot :green_circle: Document Coverage Check Passed!

[wineee]

include(GNUInstallDirs) 时会根据 CMAKE_INSTALL_PREFIX 计算 CMAKE_INSTALL_FULL_XXXX 变量的值，后面如果 CMAKE_INSTALL_PREFIX 修改 CMAKE_INSTALL_FULL_XXXX 不会随之变化，因此必须将 set CMAKE_INSTALL_PREFIX 写在前面

https://github.com/linuxdeepin/dtkcore/pull/311 https://github.com/linuxdeepin/dtkgui/pull/152 同理

[wineee]

/merge