Build shairport-sync 4.3.7 from its tag

The task passed the script `${shairport_sync_version}` - shell syntax in a Jinja field,
and the wrong variable name - which the Pi's shell expanded to nothing, so the script
fell back to its own default. Its `git reset` then ran outside the fresh clone, failed,
and was ignored without `set -e`: every build was simply upstream master. The script
now takes `pi_shairport_sync_version`, requires the argument, clones that tag directly
and stops on the first error. Bumped to 4.3.7.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

roles/pi_shairport/defaults/main.yml

@@ -1,3 +1,3 @@
 ---
-pi_shairport_sync_version: "3.3.5"
+pi_shairport_sync_version: "4.3.7"
 pi_shairport_name: Unnamed Raspberry with shairport