gfauto: print backtrace on SIGABRT
gfauto: print backtrace on SIGABRT

The `catchsegv` tool only prints a backtrace on SIGSEGV by default. We set an environment variable to ensure we also print a backtrace on SIGABRT.

google/graphicsfuzz,google/graphicsfuzz,google/graphicsfuzz,google/graphicsfuzz,google/graphicsfuzz,google/graphicsfuzz,google/graphicsfuzz