Modify package_etw.bat to not add WPT 8.1 installer
Modify package_etw.bat to not add WPT 8.1 installer

Since UIforETW no longer needs the WPT 8.1 installer we can save ~13 MB
by not including it. This change also fixes two minor installer bugs:
- %UIforETW%\ leads to sloppy double back-slashes
- flame_graph.exe was being copied from the wrong location.

mwinterb/UIforETW,google/UIforETW,mwinterb/UIforETW,mwinterb/UIforETW,ariccio/UIforETW,MikeMarcin/UIforETW,ariccio/UIforETW,ariccio/UIforETW,MikeMarcin/UIforETW,google/UIforETW,google/UIforETW,MikeMarcin/UIforETW,ariccio/UIforETW,google/UIforETW