Block step to inject ENV VARS

[jmendiara]

Currently, if I use a block step, the only way to retrieve the user values in a following step is using buildkite-agent meta-data get <key> It would be nice if these meta-data values are avaiable also as ENV vars

Example:

- block: "Request Release"
 fields:
   - text: "Code Name"
     key: "RELEASE_NAME"

The user writes "emotional dog"

would execute the next step with env var RELEASE_NAME=emotional dog

[jmendiara]

Related to #427