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ARROW-5820: [Release] Remove undefined variable check from verify script

External shell scripts may refer unbound variable:

    /tmp/arrow-0.14.0.yum2X/apache-arrow-0.14.0/test-miniconda/etc/profile.d/conda.sh:
    line 55: PS1: unbound variable

Author: Sutou Kouhei <kou@clear-code.com>

Closes #4773 from kou/release-verify-without-u and squashes the following commits:

e27270390 <Sutou Kouhei>  Remove undefined variable check from verify script