Download latest MF6 using nlmod

[dbrakenhoff]

Describe the task

When downloading MF6 we now default to the executables repo, which hasn't had a release since october 2025. This results in MF6 6.6.3 being downloaded instead of the latest (at time of writing) 6.7.0.

Should be simple enough to download everything from executables first, then download from the modflow6 repo after.

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[dbrakenhoff]

Maybe just download from modflow6 repository by default instead of executables?

I think most people are looking for the MF6 binary and not all the other stuff. If users want the other stuff, they can switch the repo to executables (with the side note that this overwrites the mf6 version if it was already downloaded from the modflow6 repo).

# new default:
def download_mfbinaries(bindir=None, version_tag="latest", repo="modflow6"):
...