Installing Global EF Tool with multiple nuget feeds setup missing mention of cli argument.

When attempting to install the global EF tool, the cli uses Nuget to pull down the tool and if you have multiple nuget sources, and/or if one of them fail to respond with the package the installation fails. The fix is to add `--ignore-failed-sources` to the command. This argument is only referenced in the `dotnet restore` documentation and issues on Github.

Adding a Note for the documentation that if a user has multiple feeds and received the `dotnet : The tool package could not be restored.` to include the argument `--ignore-failed-sources` would help to prevent searching the error message to resolve the issue.

Argument Issue Reference
https://github.com/dotnet/sdk/issues/9886

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* ID: 263f21d6-6edc-7582-e0e2-fa1c9670bcc0
* Version Independent ID: b3dd8c84-18b1-f98d-64f1-7897d1482692
* Content: [EF Core tools reference (.NET CLI) - EF Core](https://docs.microsoft.com/en-us/ef/core/miscellaneous/cli/dotnet#dotnet-ef-dbcontext-scaffold)
* Content Source: [entity-framework/core/miscellaneous/cli/dotnet.md](https://github.com/dotnet/EntityFramework.Docs/blob/master/entity-framework/core/miscellaneous/cli/dotnet.md)
* Product: **entity-framework**
* Technology: **entity-framework-core**
* GitHub Login: @bricelam
* Microsoft Alias: **bricelam**