[Backport 6.7.x] fix(rel): update helm install script (#742)

Update Helm install script for CI testing pipeline.

### Test plan
 CI


 <br> Backport 9177600 from #741

Author: sourcegraph-release-bot

scripts/ci/install-helm-env.sh

@@ -3,9 +3,8 @@
 set -euf -o pipefail
 
 ### Install Helm
-curl https://baltocdn.com/helm/signing.asc | sudo apt-key add -
-sudo apt-get update
-sudo apt-get install apt-transport-https --yes
-echo "deb https://baltocdn.com/helm/stable/debian/ all main" | sudo tee /etc/apt/sources.list.d/helm-stable-debian.list
+sudo apt-get install curl gpg apt-transport-https --yes
+curl -fsSL https://packages.buildkite.com/helm-linux/helm-debian/gpgkey | gpg --dearmor | sudo tee /usr/share/keyrings/helm.gpg > /dev/null
+echo "deb [signed-by=/usr/share/keyrings/helm.gpg] https://packages.buildkite.com/helm-linux/helm-debian/any/ any main" | sudo tee /etc/apt/sources.list.d/helm-stable-debian.list
 sudo apt-get update
 sudo apt-get install helm