allow run_env and run_args to have variables

$i specified in run_env or run_args would allow us to use different port numbers, config files, etc.
Running bb-fuzz with 3 instances of AFL++ and `run_env="--bind 808$i"` would do something like this:
```
afl-fuzz ... -- ./myapp --bind 8080
afl-fuzz ... -- ./myapp --bind 8081
afl-fuzz ... -- ./myapp --bind 8082
```