[ci] Inject host_os value when parsing DEPS file

[swift-kim]

gclient-shallow-sync.py fails if host_os is not specified during gclient_eval.Parse().

Traceback (most recent call last):
  File "src/flutter/ci/tizen/gclient-shallow-sync.py", line 55, in <module>
    sys.exit(main(sys.argv[1:]))
  File "src/flutter/ci/tizen/gclient-shallow-sync.py", line 50, in main
    local_scope = gclient_eval.Parse(deps_contents, deps_file)
  File "/usr/share/depot_tools/gclient_eval.py", line 50[9](https://github.com/flutter-tizen/engine/runs/6434530758?check_suite_focus=true#step:5:9), in Parse
    result = Exec(content, filename, vars_override, builtin_vars)
  File "/usr/share/depot_tools/gclient_eval.py", line 4[15](https://github.com/flutter-tizen/engine/runs/6434530758?check_suite_focus=true#step:5:15), in Exec
    value = _gclient_eval(node, filename, vars_dict)
  File "/usr/share/depot_tools/gclient_eval.py", line 339, in _gclient_eval
    return _convert(node_or_string)
  File "/usr/share/depot_tools/gclient_eval.py", line 276, in _convert
    return list(map(_convert, node.elts))
  File "/usr/share/depot_tools/gclient_eval.py", line 285, in _convert
    node_dict.SetNode(key, _convert(value_node), value_node)
  File "/usr/share/depot_tools/gclient_eval.py", line 276, in _convert
    return list(map(_convert, node.elts))
  File "/usr/share/depot_tools/gclient_eval.py", line 3[22](https://github.com/flutter-tizen/engine/runs/6434530758?check_suite_focus=true#step:5:22), in _convert
    raise KeyError(
KeyError: "host_os was used as a variable, but was not declared in the vars dict (file 'src/flutter/DEPS', line 739)"
Error: Process completed with exit code 1.

Also the --disable-desktop-embeddings option must be passed to gn to explicitly skip embedder build on macOS. Otherwise, the following error is generated.

The file:
  //out/linux_release_arm64/libflutter_mac_gtk.so
is listed as an input or source for the target:
  //flutter/shell/platform/linux:flutter_gtk
but no targets in the build generate that file.

Contributes to flutter-tizen/flutter-tizen#365.