add support to upgrade msys2

[pjljvandelaar]

I already use stack for a long time.
Although I am using the latest version of stack (via stack upgrade), I am not using the latest version of its dependencies.
See haskell/cabal#7494 (comment)

I was unable to update the msys2 dependency.
Examples of commands I tried are:

stack upgrade msys2
stack install msys2

In the end I uninstalled stack
and reinstalled it again. On the first run, stack started downloading the latest version of msys2!

Could support be added to stack to also upgrade its dependencies, such as msys2?

[cdornan]

@pjljvandelaar could you raise this issue in the stack issue tracker — cheers.