Fix FileExistsError on subsequent bazel builds on Windows

[NovaSagittarii]

Fixes #1181. os.rename() has different behaviors on Windows vs Linux, using os.replace() instead fixes the
issue when a new build is created when previous files exist from an older build.

I'm not sure if this is the best way to do this though since there's the method move_with_overwrite() in emsdk.py which does what os.replace() is supposed to do. Regarding atomicity, according to python documentation, os.replace()'s renaming is atomic as a POSIX requirement, but Windows isn't fully POSIX compliant.

emsdk/emsdk.py

Lines 547 to 553 in 8822664

	# On windows, rename/move will fail if the destination exists, and there is no
	# race-free way to do it. This method removes the destination if it exists, so
	# the move always works
	def move_with_overwrite(src, dest):
	if os.path.exists(dest):
	os.remove(dest)
	os.rename(src, dest)

[walkingeyerobot]

thanks very much!