dotnetcore-runtime.install: detect existing installation and do nothing

[jberezanski]

As suggested by @DarwinJS on Disqus.

[jberezanski]

https://github.com/dotnet/cli/blob/master/Documentation/specs/cli-installation-scenarios.md#detecting-dotnetcli-installation

HKEY_LOCAL_MACHINE\SOFTWARE\{Wow6432Node\}dotnet\Setup\InstalledVersions{x64|x86}\sharedfx\Microsoft.NETCore.App
x.y.z:DWORD=1

(note: always in Wow6432Node on Win64)

[jberezanski]

This has now gained a priority bump, because:

1. starting with 3.0 the native installers fail with error 0x80070666 "Cannot install a product when a newer version is installed" when attempting to install an earlier version in the same Major.Minor family;
2. starting with 5.0 the runtimes are updated via Microsoft Update and the packages on chocolatey.org will always lag a bit behind, so until the updated packages pass moderation, attempts to install the currently published packages will fail on systems which have been updated via Microsoft Update.

[jberezanski]

See: https://chocolatey.org/packages/dotnet-5.0-aspnetruntime#comment-5182697347