Allow disabling wallet build in teamcity build.sh

Summary: Allow allow to be disabled in the CI build script via an environment variable.

Test Plan: DISABLE_WALLET=true ./contrib/teamcity/build.sh

Reviewers: deadalnix, matiu, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1019