[Factor-builds] mason on windows-x86-64: build loop error

[<sl...@fa...>]

Fatal error on windows7-64

Win32 error 0x91: The directory is not empty.(U) Quotation: [ set-namestack init-catchstack self quot>> call -> stop ]
(O) Word: build-loop
(O) Word: build
(O) Word: cleanup
(O) Word: (win32-error)
(O) Word: windows-error
(O) Method: M\ object throw
(U) Quotation: [
        OBJ-CURRENT-THREAD special-object error-thread set-global
        continuation -> error-continuation set-global
        [ original-error set-global ] [ rethrow ] bi
    ]