[MCUMGR] MCUMGRPATH not recognized on Windows

[jalinei]

Describe the bug

After few successful (usually around 3) compilation and flashing action, windows stop executing mucmgr.exe correctly.
The failed command is the following (defined in pio_extra.ini):
upload_command = MCUMGRPATH $UPLOAD_FLAGS $SOURCE
The root cause seems to be in pre_bootloader_serial.py :

	env["UPLOADCMD"] = env["UPLOADCMD"].replace("MCUMGRPATH", mcumgr_path) 

somehow the environement variable MCUMGRPATH is not written properly anymore..

"MCUMGRPATH" is not recognized as an internal or external command

To Reproduce
Steps to reproduce the behavior:

1. proceed to install core on windows following the getting started tutorial
2. flash until error message pops up.

Desktop (please complete the following information):

• OS: Windows