Add CodeBuild support

[derwasp]

Add CodeBuild support based on the CODEBUILD_WEBHOOK_HEAD_REF environment variable documented here. It seems like currently, this is the only way to determine the branch name in CodeBuild.

Though the documentation states that CODEBUILD_SOURCE_VERSION for GitHub can also be

branch name, or tag name associated with the version of the source code to be built

this is what an export output shows for a build triggered by a GitHub webhook:

export CODEBUILD_AGENT_ENDPOINT='http://127.0.0.1:7831'
export CODEBUILD_AUTH_TOKEN='359a05a3-4ca9-4e45-8237-297e076628bf'
export CODEBUILD_BMR_URL='https://CODEBUILD_AGENT:3000'
export CODEBUILD_BUILD_ARN='arn:aws:codebuild:eu-west-1:527837446413:build/DZ-DELETEME:4d2cce79-1e48-4525-bfc4-22c1c0f3088b''
export CODEBUILD_BUILD_ID='DZ-DELETEME:4d2cce79-1e48-4525-bfc4-22c1c0f3088b'
export CODEBUILD_BUILD_IMAGE='aws/codebuild/standard:2.0'
export CODEBUILD_BUILD_SUCCEEDING='1'
export CODEBUILD_BUILD_URL=[REDACTED]
export CODEBUILD_CI='true'
export CODEBUILD_CONTAINER_NAME='default'
export CODEBUILD_EXECUTION_ROLE_BUILD=''
export CODEBUILD_GOPATH='/codebuild/output/src322582804'
export CODEBUILD_INITIATOR='GitHub-Hookshot/951dca1'
export CODEBUILD_KMS_KEY_ID=[REDACTED]
export CODEBUILD_LAST_EXIT='0'
export CODEBUILD_LOG_PATH='9a1704b1-6113-4858-8e28-d19b156c9da3'
export CODEBUILD_PROJECT_UUID='a419f548-3f2a-419b-aff9-becca6830831'
export CODEBUILD_RESOLVED_SOURCE_VERSION='9f74be745a0ea7b2a088581305ac0a2e8c62e173'
export CODEBUILD_SOURCE_REPO_URL='https://github.com/derwasp/somerepo'
export CODEBUILD_SOURCE_VERSION='9f74be745a0ea7b2a088581305ac0a2e8c62e173'
export CODEBUILD_SRC_DIR='/codebuild/output/src322582804/src/github.com/derwasp/somerepo'
export CODEBUILD_START_TIME='1569507961804'
export CODEBUILD_WEBHOOK_ACTOR_ACCOUNT_ID='99999999'
export CODEBUILD_WEBHOOK_EVENT='PUSH'
export CODEBUILD_WEBHOOK_HEAD_REF='refs/heads/feature/testfeature'
export CODEBUILD_WEBHOOK_PREV_COMMIT='0000000000000000000000000000000000000000'
export CODEBUILD_WEBHOOK_TRIGGER='branch/feature/testfeature'

Here the branch name is feature/testfeature and as it can be seen it's only mentioned in CODEBUILD_WEBHOOK_HEAD_REF and CODEBUILD_WEBHOOK_TRIGGER.

CODEBUILD_WEBHOOK_HEAD_REF seems to be a direct map from Github's push event's ref field.

Closes #1771

[derwasp]

@asbjornu @arturcic I would appreciate any feedback on the PR ;)

[asbjornu]

I have no idea what CodeBuild even is, so I can't review this in any meaningful way, but as the tests are green and I assume you've tested this, I'll merge as soon as the PR is up to date with HEAD of master. 😃

[derwasp]

@asbjornu I hope I tested it enough :)
CodeBuild is a very simple build server/service from AWS, similar to Azure Pipelines and others. But I guess you figured that out :)

[asbjornu]

Thank you for your contributions, @derwasp. 🙏