vendor build: pass ghostty its own version, not our tagsv0.1.0

The vendored tree has no .git, so upstream's version detection runs
git describe in *our* repository -- and the moment this repo grew its
first release tag (v0.1.0), ghostty's build refused it: 'tagged
releases must be in vX.Y.Z format matching build.zig'. Our v0.1.0 is
not ghostty's 1.3.2-dev, and upstream is right not to let a stranger's
tag name its build.

-Dversion-string is upstream's own escape hatch for exactly this. The
value is read from the vendored build.zig.zon rather than written here,
so a re-vendor updates it in the one place it already lives. Both
invocations (the host vendor target and the cross-built release legs)
get it through one VT_FLAGS.

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