CI: Extract LLVM win64 installer directly, using 7z

Currently, we have LLVM tarballs for win64, generated by someone running
the installer via wine and tarring up the result.

7z knows how to extract NSIS installers directly, and the result is
identical to our tarball, except that it doesn't include `Uninstall.exe`
(which we don't care about) and it includes the NSIS plugin directory
(which we also don't care about).

This simplifies the process of upgrading CI, and allows us to just
mirror the upstream release .exe directly. This also improves our
supply chain.

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