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