`dotnet tool exec` needs to support the `--tool-manifest` flag

[baronfel]

tool exec supports using tool manifests to supply the 'version` component of a tool package that is directly invoked - but this only works if the tool manifest is relative to the current working directory.

The --tool-manifest option is used on the other dotnet tool commands to allow explicit specification of the manifest for just this reason - exec should support it too.

This was discovered by @afscrome in dotnet/aspire#13077 (comment).