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

Angelos Evripiotisrequested to merge
bloomberg/bst/buildstream:utils_call_race into master

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 $!

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