feat: use an env var to control arch when building orbit for local tuf server (#16263)

Just a small developer quality of life update

# Checklist for submitter

If some of the following don't apply, delete the relevant line.

<!-- Note that API documentation changes are now addressed by the
product design team. -->

- [x] Manual QA for all new/changed functionality

tools/tuf/test/README.md

@@ -55,6 +55,14 @@ LINUX_TEST_EXTENSIONS="./tools/test_extensions/hello_world/linux/hello_world_lin
 ./tools/tuf/test/main.sh
 ```
 
+To build for a specific architecture, you can pass the `GOARCH` environment variable:
+``` shell
+[...]
+GOARCH=arm64 # defaults to amd64
+[...]
+./tools/tuf/test/main.sh
+```
+
 # Add new updates
 
 To add new updates (osqueryd or orbit), use `push_target.sh`.

tools/tuf/test/create_repository.sh

@@ -61,13 +61,9 @@ for system in $SYSTEMS; do
     rm $osqueryd_path
 
     goose_value="$system"
-    goarch_value="" # leave it empty to use the default for the system
+    goarch_value=${GOARCH:-}
     if [[ $system == "macos" ]]; then
         goose_value="darwin"
-	# for all platforms except Darwin, GOARCH is hardcoded to amd64 to
-	# prevent cross compilation issues when building macOS arm64 binaries
-	# from Linux (CGO + libraries are required)
-	goarch_value="amd64"
     fi
     orbit_target=orbit-$system
     if [[ $system == "windows" ]]; then