SlowBuffer RangeError when piping tar to KeyBase

keybase fails when attempting to create a large (1.5GB) tar archive and piping it to keybase encrypt.

Stack trace below:

```
╰─➤ tar -cf - large_directory | keybase encrypt nolan > large_directory.tar.asc

buffer.js:194
this.parent = new SlowBuffer(this.length);
^
RangeError: length > kMaxLength
at new Buffer (buffer.js:194:21)
at Function.Buffer.concat (buffer.js:495:16)
at BufferOutStream.exports.BufferOutStream.BufferOutStream.data (/usr/local/lib/node_modules/keybase/node_modules/iced-spawn/lib/stream.js:102:21)
at /usr/local/lib/node_modules/keybase/node_modules/iced-spawn/lib/cmd.js:484:32
at Deferrals.exports.Deferrals.Deferrals._call (/usr/local/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:86:16)
at /usr/local/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:98:26
at exports.trampoline.trampoline (/usr/local/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:64:14)
at Deferrals.exports.Deferrals.Deferrals._fulfill (/usr/local/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:96:16)
at ret (/usr/local/lib/node_modules/keybase/node_modules/iced-runtime/lib/runtime.js:29:18)
at SpawnEngine.BaseEngine._maybe_call_callback (/usr/local/lib/node_modules/keybase/node_modules/iced-spawn/lib/cmd.js:57:16)
```

Start with node_modules/keybase/node_modules/iced-spawn/lib/stream.js at BufferOutStream.data and the Buffer.concat call shown in the stack trace. Reproduce the tar-to-keybase command with a large archive, then determine a change that avoids the RangeError and verify that large streamed input can be encrypted successfully.

Written by the indexing model from the issue text.