Correct trailing path separator

In [1], I landed a change that contained a bug. It meant to trim off a
trailing path separator. Instead of trimming the end, it trimmed
everything but the first char (since the path separator is 1 char long).

This CL fixes that bug.

[1] https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3451982

Change-Id: Ie4aebfc1042908437bd603da1c20681f19c50f73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3473285
Auto-Submit: Chris Blume <cblume@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>

win_toolchain/package_from_installed.py

@@ -188,7 +188,7 @@ def BuildFileList(override_dir, include_arm):
 
   # Strip off a trailing slash if present
   if sdk_path.endswith(os.path.sep):
-    sdk_path = sdk_path[:len(os.path.sep)]
+    sdk_path = sdk_path[:-len(os.path.sep)]
 
   debuggers_path = os.path.join(sdk_path, 'Debuggers')
   if not os.path.exists(debuggers_path):