Removes unbound var in or-tools script

[daniel-j-h]

Downstream builds are failing because of bash unbound var errors.

Not sure when exactly this changed, all I could find in the changelog is

Fixed
    Unbound variables in sparsehash, bzip2, and harfbuzz packages

Can we automatically check all packages?

cc @springmeyer

[springmeyer]

Not sure when exactly this changed, all I could find in the changelog is

This started throwing in the v0.8.0 release per mason now run with set -u to catch undefined variables in scripts (#356) https://github.com/mapbox/mason/blob/master/CHANGELOG.md#080.

Can we automatically check all packages?

Kicked off this work at #408

[springmeyer]

Downstream builds are failing because of bash unbound var errors.

Note: in the future we plan to finish the lightweight client (#396), such that all downstream installers of packages will only depend on the mason.ini and therefore be isolated from any breakages in the script.sh which will only be used for builds.

[springmeyer]

Thanks for finding and fixing this bug @daniel-j-h - will merge now. This will go out in the upcoming v0.10.0 release.