Make preferring runtime env default off

[patrobinson]

Description

In v3.63.0 we made a breaking change to how pipelines were parsed, specifically the precedence of environment variables in a pipeline upload step.

This undoes that breaking change to restore the original behaviour (see buildkite/go-pipeline#35)

Changes

The following pipeline example exhibits the breaking change (Credit to @lizrabuya for the reproducible test case).

Pipeline Steps:

env:
  HEADER_ENV: "123"
​
steps:
  - command:
      - buildkite-agent pipeline upload pipeline.yml

pipeline.yml:

env:
  HEADER_ENV: "789" 
  NEW_ENV: "NEW-${HEADER_ENV:-}"
steps: 
- label: "Fixed Values ENV: ${NEW_ENV}" 
  commands: echo "Checking fixed env values ${NEW_ENV} "
- label: "Fixed Values Runtime ENV"
  commands: echo "Checking fixed env values $${NEW_ENV} "

Prior to agent version v3.63.0 this pipeline printed Checking fixed env values NEW-789 for both the first and second command jobs.
As of v3.63.0 and above the steps now prints NEW-123.

This restores the original behaviour.

This PR also adds an experiment flag interpolation-prefers-runtime-env which enables the behaviour introduced in v3.63.0.

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go fmt ./...)

[DrJosh9000]

Good find!

[DrJosh9000]

✅✅