Limit concurrent downloads by UV (#39810)

In some circumstances, the number of parallell downloads run by
UV might hit local networking or proxy limits. This PR limits the
number of concurrent downloads to number of processors that the
docker container has to avoid those problems.

(cherry picked from commit 5aee696)

scripts/in_container/_in_container_utils.sh

@@ -80,6 +80,8 @@ function in_container_get_packaging_tool() {
         fi
         export UPGRADE_EAGERLY="--upgrade --resolution highest"
         export UPGRADE_IF_NEEDED="--upgrade"
+        UV_CONCURRENT_DOWNLOADS=$(nproc --all)
+        export UV_CONCURRENT_DOWNLOADS
     else
         echo
         echo "${COLOR_BLUE}Using 'pip' to install Airflow${COLOR_RESET}"