Environment variable error appear even if variable is not used for current step

[corradio]

To reproduce:

1. add an BRICK_ env variable as part of a command in the build step
2. run the prepare step without the env variable (brick prepare)
3. brick will crash saying that the variable isn't defined

Expected behaviour:
Prepare step should run fine.

[skovhus]

We currently expand variables once for the entire config. A bit of a tradeoff.

But we haven't really had an issue with it as we usually use default values:

  commands:
      - REACT_APP_COMMIT_SHA=${BRICK_COMMIT_SHA:-unknown} yarn build

[corradio]

We currently expand variables once for the entire config. A bit of a tradeoff.

But we haven't really had an issue with it as we usually use default values:

  commands:
      - REACT_APP_COMMIT_SHA=${BRICK_COMMIT_SHA:-unknown} yarn build

good call, I'll do that!