Add option for using the staging pacman repo

[mkoeppe]

This could shorten development cycles by cutting out the delay for uploading packages that have been built in https://packages.msys2.org/queue

[lazka]

As an experimental option maybe. The packages are not signed and staging is broken from time to time.

[mkoeppe]

Sure, I would myself only use this as an option in a manually invoked Actions workflow.

[lazka]

Sounds good.

I've pushed the packages btw.