Powershell: MSIX installer doesn't install PowerShell on Windows 10

[jeroenlandheer]

Steps to reproduce

• Install powershell with the MSIX installer.

Expected behavior

• PowerShell should install

Actual behavior

• The package does not install. Error message: "This app package is not supported for installation by App Installer because it uses certain restricted capabilities."
  

Environment data

• Windows 10 Professional version 1903.
• PowerShell Core v7.0 RC1

[Zenexer]

Able to reproduce on a relatively fresh install of Windows 10 Pro 1909.

[vexx32]

See the notes in the installation documentation:

Once downloaded, you cannot simply double-click on the installer as this package requires use of un-virtualized resources. To install, you must use the Add-AppxPackage cmdlet:

Add-AppxPackage PowerShell-<version>-win-<os-arch>.msix

I think I remember that @SteveL-MSFT mentioned that this isn't something they're able to simply "resolve" for a downloaded MSIX, but will be a non-issue for releases that make it to the Windows Store proper.

Note that as mentioned in #10469, you will need to have Developer Mode enabled on your system to allow the cmdlet to be used in this way.

[Zenexer]

@vexx32 Could this be added to the README? Currently there aren't any instructions for the MSIX.