`rebar3 efmt` doesn't work with stdin

When calling `efmt` via `rebar3`, formatting through stdin doesn't work:

```sh
# This works
cat mymod.erl | efmt -
# This blocks indefinitely
cat mymod.erl | rebar3 efmt -
```

Versions:
```sh
$ rebar3 efmt -V
efmt 0.15.0
$ rebar3 version
rebar 3.19.0 on Erlang/OTP 25 Erts 13.1.5
```