Update URI for gcloud CLI distribution

[mrodm]

Update URI used to install google cloud SDK (gcloud CLI).
Current documentation uses "https" as scheme in the APT sources: https://cloud.google.com/sdk/docs/install#deb

There were also failing some Buildkite builds when running the command to build the docker image (with "http"):

2.460 W: Failed to fetch http://packages.cloud.google.com/apt/dists/cloud-sdk/main/binary-amd64/Packages 502 Bad Gateway [IP: 173.194.74.138 80]
2.460 W: Failed to fetch http://packages.cloud.google.com/apt/dists/cloud-sdk/main/binary-all/Packages
2.460 W: Some index files failed to download. They have been ignored, or old ones used instead.

https://buildkite.com/elastic/elastic-package/builds/2512#018e09df-6296-47bb-b89d-b0023bbdd022/132-900

Relates #1700

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: d2c308c

cc @mrodm