[java templates] build.sh should detect sample-secrets

[chingor13]

Many libraries use secrets from secret manager for samples. We source a file in bash which loads environment variables from a secret file.

Example: https://github.com/googleapis/java-bigquerystorage/blob/031655be10038a25dcd6285166fc3269599df525/.kokoro/build.sh#L81-L84

We should generalize this (look for all -samples-secrets files) so maintainers do not need to customize build.sh and ignore the common templates.