Downgrade prebuilt runner helper images to Alpine 3.19

In Alpine 3.21, Git appears to have some regression supplying when
supplying a bundle that doesn't exactly match the server
certificates. Previously Git would fall back to using the system
store, but this no longer appears to work.

For example, suppose you use the cert bundle from google.com
and attempt a clone from gitlab.com:

```
openssl s_client -showcerts -connect google.com:443 </dev/null | openssl x509 -outform PEM > google.pem
git config --global http.sslCAInfo google.pem
git clone https://gitlab.com/group/project
```

In Alpine 3.21, this fails with `SSL certificate problem: self-signed
certificate in certificate chain`.

In Alpine 3.18 - 3.20, this works fine.

Revert to Alpine 3.19 until this issue is resolved:
https://gitlab.alpinelinux.org/alpine/aports/-/issues/16980

Relates to https://gitlab.com/gitlab-org/gitlab-runner/-/issues/38644

Changelog: fixed

Author: stanhu

Makefile.runner_helper.mk

@@ -79,7 +79,7 @@ out/helper-images:
 	cd dockerfiles/runner-helper && docker buildx bake --progress plain $(TARGETS)
 
 .PHONY: prebuilt-helper-images
-prebuilt-helper-images: ALPINE_DEFAULT_VERSION=3.21
+prebuilt-helper-images: ALPINE_DEFAULT_VERSION=3.19
 prebuilt-helper-images:
 	@find out/helper-images -maxdepth 1 -name "*.tar" | parallel -j$(shell nproc) './ci/prebuilt_helper_image {}'