[🐛 BUG]: Default value support for RR config env variables

[benalf]

No duplicates 🥲.

• I have searched for a similar issue in our bug tracker and didn't find any solutions.

What happened?

Setting a default value for a RR value via envvar substitution in the RR config seems to break the value and it becomes an empty string instead.

Version (rr --version)

rr version 2.12.3 (build time: 2023-02-16T13:08:41+0000, go1.20), OS: linux, arch: amd64

How to reproduce the issue?

E.g.

.rr.yaml
______
version: '2.7'
rpc:
  listen: tcp://127.0.0.1:${RPC_PORT:-6001}
 
or

version: '2.7'
rpc:
  listen: tcp://127.0.0.1:${RPC_PORT:6001}
______
./rr serve
rpc         	plugin was started	{"address": "tcp://127.0.0.1:"    <----- missing port
______
RPC_PORT=6666 ./rr serve
rpc         	plugin was started	{"address": "tcp://127.0.0.1:"    <----- missing port

The port is missing completely in both cases.

Env variables without a default work fine:

.rr.yaml
______
version: '2.7'
rpc:
  listen: tcp://127.0.0.1:${RPC_PORT}
______
./rr serve
rpc         	plugin was started	{"address": "tcp://127.0.0.1:"      <----- missing port as expected
______
RPC_PORT=6666 ./rr serve
rpc         	plugin was started	{"address": "tcp://127.0.0.1:6666"  <----- port is set as expected

Relevant log output

No response