Fix unbound SHARE_DIR variable in installer

After relocating binaries to INSTALL_ROOT, the SHARE_DIR variable was removed
but one reference remained in cache_installer_for_self_heal() causing
'unbound variable' error.

Changed to use INSTALL_ROOT directly since that's where the cached installer
is stored (STORED_INSTALLER=${INSTALL_ROOT}/install-sensor-proxy.sh).

scripts/install-sensor-proxy.sh

@@ -269,7 +269,7 @@ determine_installer_ref() {
 
 cache_installer_for_self_heal() {
     INSTALLER_CACHE_REASON=""
-    install -d "$SHARE_DIR"
+    install -d "${INSTALL_ROOT}"
 
     local source_issue=""
     if [[ -n "$SCRIPT_SOURCE" && -f "$SCRIPT_SOURCE" ]]; then