install_path parameter breaks in WINDOWS subprocesses

[maksymiuks]

When running the covr in the callr subprocess on Windows, covr::package_coverage unexpectedly breaks if install_path has double backslashes (valid WINDOWS path) \\ instead of the single forward slashes /. That's due to the path normalization which happens after creating the directory:

covr/R/covr.R

Lines 413 to 415 in ca4241d

	dir.create(install_path)
	# tools::testInstalledPackage requires normalized install_path (#517)
	install_path <- normalize_path(install_path)

Swapping these two lines will make the internal normalize_path function first replace all the \\ with / so the function can proceed as expected. I'm not sure why exactly this is an issue in a subprocess, but the change looks reasonable either way.