ci: workspace mcpp bootstrap pin -> 2026.9.11.4 (#616) #1650

Job	Run time
mingw-cross linux→windows (build + wine run)	7m 16s
windows→linux cross-build (windows host)	8m 50s
cross-build aarch64-linux-musl (mcpp + xlings)	9m 41s
windows→linux artefact really runs (linux)	4s
	25m 51s

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