Refactor MinGW cross-compilation toolchain build into a separate workflow #261

Job	Run time
mingw-cross-toolchain / mingw-w64-cross-headers / build	2m 58s
mingw-cross-toolchain / mingw-w64-cross-binutils / build	25m 3s
mingw-cross-toolchain / mingw-w64-cross-gcc-stage1 / build	55m 34s
mingw-cross-toolchain / mingw-w64-cross-windows-default-manifest / build	2m 48s
mingw-cross-toolchain / mingw-w64-cross-crt / build	11m 28s
mingw-cross-toolchain / mingw-w64-cross-winpthreads / build	3m 36s
mingw-cross-toolchain / mingw-w64-cross-gcc / build	1h 8m 45s
mingw-cross-toolchain / mingw-w64-cross-zlib / build	2m 19s
repository	1m 28s
deploy	0s
check	0s
	2h 53m 59s

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