VS2026 ships on the prerelease channel; vswhere omits it unless
-prerelease is given. Without it _find_msbuild_via_vswhere returned
nothing, so the path fallback picked an incomplete
(x86)\Microsoft Visual Studio\18\BuildTools install (MSBuild present
but no VC v180 targets) -> MSB4019 on hosts that have VS2026 IDE but
not BuildTools. -prerelease is harmless on the CI VM (BuildTools is
found regardless).
Simone
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