fix(apple): Expose MACOSX_DEPLOYMENT_TARGET in rust apple build script to signal to rustc which macOS to target (#7443)

`MACOSX_DEPLOYMENT_TARGET` is a standard env var read by gcc and rustc
that determines which version of macOS to target binaries for. This
variable was being removed inadvertently in our rust build script.

Exposing this var fixes a slew of warnings about object files being
built for a newer macOS target than being linked that were showing up in
Xcode for some time now.

Hasn't caused any issues thus far from what I can tell, but possibly
related to #7442

rust/connlib/clients/apple/build-rust.sh

@@ -12,6 +12,7 @@ cmd=${1:-""}
 # into our highly evolved Rust-based build system.
 for var in $(env | awk -F= '{print $1}'); do
     if [[ "$var" != "HOME" ]] &&
+        [[ "$var" != "MACOSX_DEPLOYMENT_TARGET" ]] &&
         [[ "$var" != "USER" ]] &&
         [[ "$var" != "LOGNAME" ]] &&
         [[ "$var" != "TERM" ]] &&