Fix the directory to execute tar in build_posix_local

I've tried to execute "scripts/build.py posix-local" on CentOS 7.
But it failed by errors with the following output.
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tar: wkhtmltox-0.12.1: Cannot stat: No such file or directory
tar: Exiting with failure status due to previous errors
tar -c -v -f ../wkhtmltox-0.12.1_local-centos7.tar wkhtmltox-0.12.1/
command failed: exit code 512
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This pull request changes the directory to execute tar.