fly pg create not setting a DATABASE_URL secret

[fliepeltje]

There's a mismatch in behavior between fly launch and fly pg create when setting up a database.

If during fly launch I specify that I want postgres, a DATABASE_URL will be set automatically.
When I run fly pg create and go through the prompts, I have to set the DATABASE_URL manually.

For consistency it owuld be nice if the env var gets set in situation 2 as well.