controllers: store HelmChart Artifact with suffix

[hiddeco]

This adds a Unix suffix to the HelmChart Artifact filename, to ensure it is unique for sequential builds triggered due to e.g. a controller restart.

The result of this is that consumers who think they are fetching an Artifact with a certain checksum run into a 404 when attempting to download a previously advertised but now unavailable file, instead of running into a checksum validation error (due to non-repetitive Helm builds).

Potentially fixes #610

[hiddeco]

@aryan9600 you probably want to have a look at this, and make suggestions if it does not combine well with your proposed changes in #605.