utils._call: fix race condition on 'process' var

Tested by inflating the chance of hitting the race. First, insert a
sleep between opening the terminator context and starting the process,
then:

    python3 -c 'import buildstream.utils; import os; \
        buildstream.utils._call(["echo", "hello"], True); \
        print(os.getpid())' & sleep 1; kill $!