Windows LAMMPS shortcuts point to C:\Windows\System32 when "Documents" is unusually located

[srtee]

See https://matsci.org/t/lammps-gui-test-version-for-macos-windows-and-linux/49921/28

Currently LAMMPS shortcuts on Windows are pointed to the initial working directory %HOMEDRIVE%%HOMEPATH%\Documents:

lammps-packages/mingw-cross/installer/lammps-gui.nsis

Line 310 in d55e3f7

SetOutPath "%HOMEDRIVE%%HOMEPATH%\Documents"

This is fragile (for example, my computer has Documents pointing to a network share drive, and anecdotally different language localizations for Windows use different names e.g. "Dokumente".

This could be fixed by either:

• using the NSIS Constant "$DOCUMENTS" (https://nsis.sourceforge.io/Docs/Chapter4.html)
• finding the My Documents folder using PowerShell: [Environment]::GetFolderPath([Environment+SpecialFolder]::MyDocuments) (https://stackoverflow.com/questions/3492920/is-there-a-system-defined-environment-variable-for-documents-directory) and then putting that into environment variables.