ci: bootstrap pin moves to 2026.9.2.1 #1281

Job	Run time
mingw-cross linux→windows (build + wine run)	6m 12s
cross-build aarch64-linux-musl (mcpp + xlings)	7m 35s
windows→linux cross-build (windows host)	8m 47s
windows→linux artefact really runs (linux)	5s
	22m 39s

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