Revert "Revert "Revert "Add path interposer"""

This reverts commit c59a92cb1a.

Reason for revert: tests are broken with long OUT_DIRs
They're directly calling net.Listen, and not using the fallback
for long socket names.

Change-Id: Id14cbd499fd9b36c6926b7552d3554340cb0916c

ui/build/build.go

@@ -140,8 +140,6 @@ func Build(ctx Context, config Config, what int) {
 
 	ensureEmptyDirectoriesExist(ctx, config.TempDir())
 
-	SetupPath(ctx, config)
-
 	if what&BuildProductConfig != 0 {
 		// Run make for product config
 		runMakeProductConfig(ctx, config)