BridgeDB silently ignores RUN_IN_DIR option when run with --dump-bridges

BridgeDB silently ignores the RUN_IN_DIR option when being run with the --dump-bridges option. The result is that it doesn't find its database and dumps no bridges to files. I don't know if the fix is to respect the RUN_IN_DIR option, or if we should respect more config options when running with the --dump-bridges option.