feat: allow overriding desktop and distro for tests

[adil192]

In a test for my app, I wanted to have platform.isUbuntu be true. But there wasn't an easy way to set the distro without specifying the entire os-release file.

This PR adds PlatformLinuxDistro.osIdOverride and PlatformLinuxDesktop.xdgCurrentDesktopOverride so users of the platform_linux package can override the distro/desktop.

Before this PR:

IOOverrides.runZoned(
  () {
    final platform = FakePlatform(
      environment: {
        'XDG_CURRENT_DESKTOP': 'GNOME',
      },
    );
  },
  createFile: MockTextFiles({
    '/etc/os-release': MockTextFile('''
PRETTY_NAME="Ubuntu 23.04"
NAME="Ubuntu"
VERSION_ID="23.04"
VERSION="23.04 (Lunar Lobster)"
VERSION_CODENAME=lunar
ID=ubuntu
ID_LIKE=debian
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
UBUNTU_CODENAME=lunar
LOGO=ubuntu-logo
'''),
  }).call,
);

After this PR:

final platform = FakePlatform();
platform.xdgDesktopOverride = ['gnome'];
platform.distroOverride = 'ubuntu';