GoReleaser Pro --split defaults to partitioning by GOOS only, so each
linux matrix runner built both linux/amd64 and linux/arm64. With
CGO_ENABLED=1 (go-tree-sitter), the native gcc cannot assemble the other
arch's CGO runtime (gcc_arm64.S: no such instruction), failing the build.
- .goreleaser.yaml: set partial.by=target (partition by GOOS+GOARCH)
- workflow-prepare.sh: export GGOOS/GGOARCH from the matrix TARGET_* vars
so each native runner builds exactly its own target
GGOOS/GGOARCH are filter-only and do not bleed into before-hooks, unlike
GOOS/GOARCH.
