`dotnet tool install` says paket 6.0.0-alpha042 is not compatible with net50

Description

PS C:\Users\rei> dotnet tool install --global paket --version 6.0.0-alpha042
error NU1202: Package Paket 6.0.0-alpha042 is not compatible with net50 (.NETFramework,Version=v5.0) / any. Package Paket 6.0.0-alpha042 supports: netcoreapp2.1 (.NETCoreApp,Version=v2.1) / any
The tool package could not be restored.
Tool 'paket' failed to install. This failure may have been caused by:

* You are attempting to install a preview release and did not use the --version option to specify the version.
* A package by this name was found, but it was not a .NET tool.
* The required NuGet feed cannot be accessed, perhaps because of an Internet connection problem.
* You mistyped the name of the tool.

For more reasons, including package naming enforcement, visit https://aka.ms/failure-installing-tool

Repro steps

1. Install latest SDK
2. Check dotnet version:

PS C:\Users\rei> dotnet --version
5.0.100-rc.1.20452.10

3. Attempt to install paket as a dotnet tool, either with the latest version (which resolves to 5.249.2) or by specifying 6.0.0-alpha042:

PS C:\Users\rei> dotnet tool install --global paket --version 6.0.0-alpha042

Expected behavior

Installs paket for .NET prerelease.

Actual behavior

Error message as noted above.

Known workarounds

Start by reproducing the reported dotnet tool install --global paket --version 6.0.0-alpha042 failure with SDK version 5.0.100-rc.1.20452.10, then inspect the Paket tool package's target framework metadata. Done means the prerelease Paket tool installs successfully for the reported .NET SDK scenario.