update secrets plugin submodule

The full diff is at [1]. It's mostly docs, improved logging and tidy
ups with no impact on behaviour. The one exception is we now set the
AWS_METADATA_SERVICE_NUM_ATTEMPTS environment variable, so the awscli
will retry failed instance metadata requests a few times.

In theory instance metadata requests are very likely to fail, however
with enough instances across enough customers.... we have seen it
happen. The failures are intermittent and temporary, so hopefully retries
will help. More details at [2]

[1] https://github.com/buildkite/elastic-ci-stack-s3-secrets-hooks/compare/2a11ab5...48c2116e91850d94aec155f8e8870a4c687e78d7?w=1
[2] buildkite/elastic-ci-stack-s3-secrets-hooks#30

Author: yob