Only remove build/ if exists

Signed-off-by: David Goulet <dgoulet@torproject.org>

build.sh

@@ -78,7 +78,9 @@ SIGS_DIRPATH="$(pwd)/sigs"
 
 # Cleanup previous build directory since we need to start from scratch. Prompt
 # before removal just in case since rm -rf are scary.
-runcmd rm -rf -I "$BUILD"
+if [ -d "$BUILD" ]; then
+    runcmd rm -rf -I "$BUILD"
+fi
 
 # Create the directory hierarchy we need.
 runcmd mkdir -p "$BUILD"