Building installer fails on Windows 10 Msys2
Description of problem:
build fails with:
```
make[2]: Leaving directory '/c/Users/sxlga/source/repos/qemu/build'
Traceback (most recent call last):
  File "C:\Users\sxlga\source\repos\qemu\scripts\nsis.py", line 89, in <module>
    main()
  File "C:\Users\sxlga\source\repos\qemu\scripts\nsis.py", line 34, in main
    with open(
OSError: [Errno 22] Invalid argument: 'C:/Users/sxlga/AppData/Local/Temp/tmpinyvlwkoC:/msys64/qemu/system-emulations.nsh'
ninja: build stopped: subcommand failed.
make[1]: *** [Makefile:165: run-ninja] Error 1
make[1]: Leaving directory '/c/Users/sxlga/source/repos/qemu/build'
make: *** [GNUmakefile:11: installer] Error 2
```
Steps to reproduce:
1. ./configure --target-list=arm-softmmu,aarch64-softmmu
2. make all
3. make installer
Additional information:
following https://wiki.qemu.org/Hosts/W32#Native_builds_with_MSYS2 to set up toolchain