Honor again OBS_REBUILD_DISABLED=1

After the move to the dedicated OBS runner setting OBS_REBUILD_DISABLED=1 lost its effectiveness.

From https://docs.gitlab.com/ee/ci/yaml/#rules:

Rules are evaluated when the pipeline is created, and evaluated in order until the first match.

This means that since the preceding rule matches to cause the execution of the job, the one setting REBUILD_VALUE=false gets ignored.

Move that setting to the build.env file to work around the issue.