Follow-up to 0453438d. Previously, setting CARGO_BUILD_TARGET before
manually calling build_jni.sh merely did a Cargo build with the same
settings as the host build. However, adding support for cross-compiles
for the server build broke this "ad hoc" cross-compilation.
Fortunately, it was used only in one place: providing both flavors of
Mac library in a built libsignal-client and libsignal-server. We can
use the *new* cross-compilation support for this instead, reducing
special cases.
This also limits the conditions where build_jni.sh will automatically
set CC and other environment variables, and allows that behavior to be
overridden by manually setting them.
d3adfba263