[Bootstrap] Force-enable the backtracer.

This *might* break things, but I need it to debug a test.

rdar://160273124

Author: al45tair

Utilities/bootstrap

@@ -907,6 +907,7 @@ def get_swiftpm_env_cmd(args):
         env_cmd.append("SWIFTCI_DISABLE_SDK_DEPENDENT_TESTS=YES")
 
     env_cmd += [
+        "SWIFT_BACKTRACE=enable=yes,warnings=suppressed",
         "SWIFT_EXEC=" + args.swiftc_path,
         "SWIFT_DRIVER_SWIFT_EXEC=" + args.swiftc_path,
         "CC=" + args.clang_path,