v2.0.0: Go go gadget 🦾

Big change:

• 10–100x speed-up: s3secrets-helper written in Go #37 (pda)

There are many other changes since the last tagged release, but they have long since been submoduled into https://github.com/buildkite/elastic-ci-stack-for-aws … they're listed here anyway:

Other changes since last tagged release:

• 🧹 early exit instead of indentation, unused grep_secrets removed. #35 (pda)
• Don't exit if can't check env file #34 (shevaun)
• Print more output around downloading environment files #33 (shevaun)
• ssh_private_key -> private_ssh_key, misc cleanup #31 (jramos-io)
• add AWS_METADATA_SERVICE_NUM_ATTEMPTS env var #30 (TheDen)
• Warn instead of fail when no SSH keys found in S3 #32 (pda)
• fix typo in README #28 (OliverKoo)
• Show unexpected errors from head-object #27 (lox)
• eval ssh-agent -k #22 (sj26)
• Fix broken git-credential test in CI #23 (lox)
• lint scripts and improve consistency #20 (TheDen)
• Updated docker-compose.yml to pass YAML Lint #12 (mgrlabs)
• Replaced '/dev/stdout' with '-' for GfW Bash #11 (mgrlabs)
• Changed output from '/dev/stdout' to '-' #10 (mgrlabs)