flang_win-64_activate.bat mixes path separators

[lucascolley]

flang-activation-feedstock/recipe/activate.bat

Lines 13 to 21 in b7c1513

	set "LDFLAGS=%LDFLAGS% -Wl,-defaultlib:%CONDA_PREFIX:\=/%/lib/clang/@MAJOR_VER@/lib/windows/clang_rt.builtins-x86_64.lib"
	:: need to distinguish how we populate `-I` based on whether we're using conda build or not;
	:: LIBRARY_INC is not available if not, but we cannot use CONDA_PREFIX unconditionally either,
	:: as that points to the wrong environment (build instead of host) when using conda-build.
	if not "%CONDA_BUILD%" == "" (
	set "FFLAGS=%FFLAGS% -I%LIBRARY_INC%"
	) else (
	set "FFLAGS=%FFLAGS% -I%CONDA_PREFIX%\Library\include"

rattler-build complains about this, see scipy/scipy#24838

[lucascolley]

cc @h-vetinari