dockerized-impala-bootstrap-and-test.sh ignores existing value of IMPALA_TOOLCHAIN_HOST

Impala build driver scripts allow changing the source location for toolchain downloads by supplying an initial value for IMPALA_TOOLCHAIN_HOST, which is evaluated in bin/impala-config.sh.
bin/jenkins/dockerized-impala-bootstrap-and-test.sh uses a slightly different, two-phase mechanism for initializing the build environment, which does not preserve the initial value of this environment variable, making the override ineffective.