Update backport-base.yml to quote branch names

[baronfel]

It's kind of hard to see the branch name in question in the error when you typo a branch name:

So if we quote the branch names being used instead in the error messages, that might be a more clear signal?

To double check:

• The right tests are in and the right validation has happened. Guidance: https://github.com/dotnet/arcade/blob/main/Documentation/Validation.md

[edvilme]

Maybe adding bold or backticks ` could make them stand out more? (on the first string. Not sure if others are parsed as markdown?)

[baronfel]

@edvilme I thought about that a bit, but for the errors reported during the CI run I'm not familiar with what syntax is allowed. The message that's posted back to the requesting PR could definitely be formatted more strongly, though.