Validate should download tarballs by default on windows.

Currently running ./validate on windows fails with

configure: Checking for Windows toolchain tarballs...
SHA256SUMS doesn't exist; have you fetched?

Error:
Needed msys2 tarballs are missing. You have a few options to get them,

  * run configure with the --enable-tarballs-autodownload option

  * run mk/get-win32-tarballs.py download x86_64

  * manually download the files listed in ./missing-win32-tarballs and place
    them in the ghc-tarballs directory.

Downloading them in advance doesn't work by default as ./validate deletes them on startup by defaults.

There is workaround. We can run configure beforehand, and run ./validate with --no-clean. But that shouldn't be required.

NB: Is there still a general build system label? I gave it CI for now.

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