Substitute environment variables in a string

## Usage

`circleci env subst [string] [flags]`

## Flags

Global flags: `-c, --config`, `--debug`, `--no-color`, `-q, --quiet` — see `circleci --help`.

## Examples

- Substitute a single variable: 
  `export API_URL=https://circleci.com`
- `circleci env subst "Base URL: $API_URL"`
- Substitute variables in a JSON payload via stdin: 
  `export TOKEN=abc123`
- `echo '{"token": "$TOKEN"}' | circleci env subst`
- Substitute into a config file before uploading: 
  `circleci env subst < .circleci/config.template.yml > .circleci/config.yml`

## Details

Substitute environment variables in a string, similar to the POSIX envsubst utility.

Pass the string as an argument, or pipe it through stdin. The command writes
the substituted result to stdout with no trailing newline added.

Supports $VAR and ${VAR} syntax. References to unset variables are replaced
with an empty string.

