MSVC differences among Jenkins gerrit_windows builders

[Stephan Bergmann]

It appears that different Jenkins build machines have different versions 
of MSVC installed, causing poor results like 
<https://gerrit.libreoffice.org/c/core/+/108878> "Upgrade external/boost 
to latest Boost 1.75.0" building green on tb77 with

> export CXX=C:/PROGRA~2/MIB055~1/2019/COMMUN~1/VC/Tools/MSVC/1427~1.291/bin/Hostx64/x86/cl.exe

(<https://ci.libreoffice.org/job/gerrit_windows/85166/>), while e.g. 
<https://ci.libreoffice.org/job/gerrit_windows/85217/>, building an 
unrelated Gerrit change that happened to be based atop that "Upgrade 
external/boost to latest Boost 1.75.0" but not atop the meanwhile fix 
<https://git.libreoffice.org/core/+/a20a464bcfe6408ddd6c66ac43e9d25d3e530cd3%5E%21> 
"external/libebook: Missing include (for std::min)", failed on tb73 with

> export CXX=C:/PROGRA~2/MIB055~1/2019/COMMUN~1/VC/Tools/MSVC/1425~1.286/bin/Hostx64/x86/cl.exe