cannot use default values for environment variables

## Bug Report

**Describe the bug**
We have a requirement where we have to add certain fields to each logs using env variable and if env variable is not set then set a default value. However if we use `${LOG_TYPE:-docker-collector-logs}` in filter we get error saying [env] variable `${LOG_TYPE:-docker-collector-logs}` is used but not set and we have a core dump

**To Reproduce**
- Steps to reproduce the problem:
1. Create a config as below
```
[SERVICE]
Flush 5
Daemon Off
Log_Level info
Plugins_File plugins.conf
HTTP_Server Off
HTTP_Listen 0.0.0.0
HTTP_Port 2020

[INPUT]
Name cpu
Tag cpu.local
Interval_Sec 1

[FILTER]
Name modify
Match *
Add type ${LOG_TYPE:-docker-collector-logs}

[OUTPUT]
Name stdout
Match *
```
2. set env variable as below
`export LOG_TYPE=test-type`
3. run fluentbit with this config
**Expected behavior**
The environment variable should be used with set value or the default value should be used. Please refer [this](https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html). Even tried `${LOG_TYPE:=docker-collector-logs}` but that does not work either

We get the error as below
```
[2022/05/31 10:06:28] [ warn] [env] variable ${LOG_TYPE:=docker-collector-logs} is used but not set
[2022/05/31 10:06:28] [ info] [engine] started (pid=8)
[2022/05/31 10:06:28] [ info] [storage] version=1.1.6, initializing...
[2022/05/31 10:06:28] [ info] [storage] in-memory
[2022/05/31 10:06:28] [ info] [storage] normal synchronization mode, checksum disabled, max_chunks_up=128
[2022/05/31 10:06:28] [ info] [cmetrics] version=0.2.2
[2022/05/31 10:06:28] [error] [filter:modify:modify.0] Invalid operation Add : type in configuration
[2022/05/31 10:06:28] [error] [lib] backend failed
[2022/05/31 10:06:28] [error] Failed initialize filter modify.0
[2022/05/31 10:06:28] [ info] [input] pausing cpu.0
```
Version Used : **1.9.3**

Please note that sometimes the env variables may not be passed and in such cases we would want to use default values (esp for b/w compatibility use cases for logging sidecars when we want to add new fields).