build-subsets.sh skips cleanup of an existing temporary source directory

[dajiaohuang]

build-subsets.sh appears to skip cleanup when the temporary source directory already exists:

if ! [ -d "$TEMP_SOURCE" ]; then
    rm -rf $TEMP_SOURCE
fi
mkdir $TEMP_SOURCE

When source/temp is left by an interrupted or previously failed run, the condition is false, so the directory is not removed and the following mkdir fails. Please invert the condition (or remove the directory unconditionally before recreating it) and add a repeat-build regression check. The relevant lines are 169-175 on the current default branch.