Fix C2664 errors (#15714)

## Summary of the Pull Request
Fix C2664 errors under latest compiler.

## References and Relevant Issues
#15309

## Detailed Description of the Pull Request / Additional comments
- Latest compilers are more strict
- Internal background of change:
[DevDiv:1810844](https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1810844)

## Validation Steps Performed
- Now successfully builds under VS `17.8.0 Preview 1.0 `
- Still successfully builds under VS `17.6.5`

## PR Checklist
- [x] Closes #15309
- [ ] Tests added/passed
- [ ] Documentation updated
- If checked, please file a pull request on [our docs
repo](https://github.com/MicrosoftDocs/terminal) and link it here: #xxx
- [ ] Schema updated (if necessary)

---------

Co-authored-by: Dan Albrecht <[email protected]>

src/cascadia/TerminalApp/TabManagement.cpp

@@ -434,7 +434,7 @@ namespace winrt::TerminalApp::implementation
  // environment variables, but the user might have set one in
  // the settings. Expand those here.
 
- path = { wil::ExpandEnvironmentStringsW<std::wstring>(path.c_str()) };
+ path = winrt::hstring{ wil::ExpandEnvironmentStringsW<std::wstring>(path.c_str()) };
  }
 
  if (!path.empty())

src/cascadia/TerminalControl/TermControl.cpp

@@ -906,17 +906,17 @@ namespace winrt::Microsoft::Terminal::Control::implementation
  if (HRESULT_FROM_WIN32(ERROR_FILE_NOT_FOUND) == hr ||
  HRESULT_FROM_WIN32(ERROR_PATH_NOT_FOUND) == hr)
  {
- message = { fmt::format(std::wstring_view{ RS_(L"PixelShaderNotFound") },
- (_focused ? _core.FocusedAppearance() : _core.UnfocusedAppearance()).PixelShaderPath()) };
+ message = winrt::hstring{ fmt::format(std::wstring_view{ RS_(L"PixelShaderNotFound") },
+  (_focused ? _core.FocusedAppearance() : _core.UnfocusedAppearance()).PixelShaderPath()) };
  }
  else if (D2DERR_SHADER_COMPILE_FAILED == hr)
  {
- message = { fmt::format(std::wstring_view{ RS_(L"PixelShaderCompileFailed") }) };
+ message = winrt::hstring{ fmt::format(std::wstring_view{ RS_(L"PixelShaderCompileFailed") }) };
  }
  else
  {
- message = { fmt::format(std::wstring_view{ RS_(L"UnexpectedRendererError") },
- hr) };
+ message = winrt::hstring{ fmt::format(std::wstring_view{ RS_(L"UnexpectedRendererError") },
+  hr) };
  }
 
  auto noticeArgs = winrt::make<NoticeEventArgs>(NoticeLevel::Warning, std::move(message));