Relaunching as admin for msixmgr depends on having msixmgr.exe in %PATH%.

Project
Win7Msix

Describe the bug
We've had a few reports recently that the installation of msix packages wasn't working on windows 7. The reported error was "Windows cannot find 'msixmgr.exe'. Make sure you typed the name correctly, and then try again." when launching the msix file. I investigated further and found out that because environment variables are inherited when launching sub processes, if you install msixmgr from any browser (Chrome in this instance), and then without restarting it try to install the msix package the error would occur.

To Reproduce

1. On Windows 7 open chrome or edge browser
2. Download and install msixmgr without closing the browser
3. Download / launch any msix package

Expected behavior
The msix package installation is successfully initiated.

Screenshots

Platform
Windows 7 SP1

Additional context
I think the problem is in how relaunching as admin is implemented.
https://github.com/microsoft/msix-packaging/blob/e00e737ba572fdedf561b0f246bac56821f95603/MsixCore/msixmgr/msixmgr.cpp#L103-L108

The issue is that lpFile is not the full path to msixmgr.exe. Since the exe is already executing we can use GetModuleFileNameW as it is done here: https://github.com/microsoft/msix-packaging/blob/e00e737ba572fdedf561b0f246bac56821f95603/MsixCore/msixmgr/msixmgr.cpp#L35-L36 to obtain full path and use it instead.

Start in MsixCore/msixmgr/msixmgr.cpp, especially the relaunch-as-admin code at lines 103-108 and the existing GetModuleFileNameW use near lines 35-36. Reproduce the Windows 7 browser-install scenario, then verify that relaunching works without requiring msixmgr.exe in the inherited PATH.