fix(nsis): output files sometimes locked by virus scanners (#5005)

[berkon]

This fixes an issue which I've observed (and documented in issue #5005) with a McAfee virus scanner: After NSIS is writing the first part of the EXE, the virus scanner locks the file for further writing for a while, which causes the second write to fail and makes it impossible to build on my work laptop, because I'm not allowed to exclude certain files from the scanner. This fix checks whether the file is writable. If not it waits and checks again every 2s until the file is released by the scanner. A corresponding message is written to the console.