kustomize_substitutions in tilt-settings.json is broken

[ncdc]

What steps did you take and what happened:

1. Try to use kustomize_substitutions for variables that you do not set as environment variables
2. These placeholders are replaced with the empty string

What did you expect to happen:

1. Successful replacement from kustomize_substitutions from tilt-settings.json

Anything else you would like to add:
The change to use kustomize with envsubst (#3361) accidentally broke this. We can maybe try to copy all the substitutions into os.environ before running kustomize.

/kind bug

[vincepri]

/milestone v0.3.x

Chatting with @wfernandes, I'd like to see a clusterctl generic command that

• Takes any files as input, uses our internal envsubst implementation, and prints out the result.
• It has a --variables flag to print out all the variables
  • This can be used to look at the defaults, print out all the required variables

[ncdc]

/assign
/lifecycle active
FYI @alexeldeib