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 <danalb@ntdev.microsoft.com>

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));