Feature Request: It would be useful to be able to specify the root temporary directory.

When running in a container, it might be the case that there is a mounted path which is faster than the overlay filesystem for handling throwaway files.
When copying/overwriting an original file (or when the --output option is used that target folder), if the source and destination is on the same volume, it wouldn't have to do any actual data copying - especially if CoW were employed.

I think the existing `DirectoryService` could be augmented to take some options where the base temp directory would default to `Path.GetTempPath()` if it wasn't set to something else.
https://github.com/dotnet/sign/blob/f4efed9e8fb3296f29497b90feb6548e506f2078/src/Sign.Core/FileSystem/DirectoryService.cs#L25

It might be possible just now to override the environment variables used when running the command to override where it thinks `Temp` is, but I feel it should be built into the CLI, rather than an external workaround.

I would suggest -tmp or --temp-directory as the command line option name.

Version: 0.9.1-beta.23530.1+f4efed9e8fb3296f29497b90feb6548e506f2078