Runtime.ExitError: Cannot create directory ‘/var/task/var’: Read-only file system

I've deployed this to Lambda, but I get an error when invoking the function.
It looks like the issue is `mkdir`. How can I change the path.

CloudWatch
```
START RequestId: aa29cf83-7b5f-47a4-947d-4f3db5fdf179 Version: $LATEST
++ dirname /var/task/bootstrap
+ BASE=/var/task
+ EXE=/var/task/bin/hello_world_lambda
+ HOME=/tmp
+ export HOME
+ export RELEASE_READ_ONLY=true
+ RELEASE_READ_ONLY=true
+ export LD_LIBRARY_PATH=/lib64:/usr/lib64:/var/runtime:/var/runtime/lib:/var/task:/var/task/lib:/opt/lib:/var/task/lib/elixir_runtime-0.1.0/priv
+ LD_LIBRARY_PATH=/lib64:/usr/lib64:/var/runtime:/var/runtime/lib:/var/task:/var/task/lib:/opt/lib:/var/task/lib/elixir_runtime-0.1.0/priv
+ /var/task/bin/hello_world_lambda foreground
mkdir: cannot create directory ‘/var/task/var’: Read-only file system
END RequestId: aa29cf83-7b5f-47a4-947d-4f3db5fdf179
REPORT RequestId: aa29cf83-7b5f-47a4-947d-4f3db5fdf179 Duration: 315.32 ms Billed Duration: 400 ms Memory Size: 128 MB Max Memory Used: 13 MB
RequestId: aa29cf83-7b5f-47a4-947d-4f3db5fdf179 Error: Runtime exited with error: exit status 1
Runtime.ExitError
```

Invoking from CLI
```
aws lambda invoke \
--function-name HelloWorld \
--region eu-north-1 \
--log-type Tail \
--payload '{"msg": "a fake request"}' \
outputfile.txt
```

output.txt
```
> cat outputfile.txt

{"errorType":"Runtime.ExitError","errorMessage":"RequestId: 403e99d9-896d-4541-a913-0c45ac7debac Error: Runtime exited with error: exit status 1"}%
```

hello_world_lambda.ex
```elixir
defmodule HelloWorldLambda do
@moduledoc """
Documentation for HelloWorldLambda.
"""

@doc """
Hello world.

## Examples

iex> HelloWorldLambda.hello()
:world

"""
def hello(request, context) do
"""
Hello World!
Request: #{Kernel.inspect(request)}
Context: #{Kernel.inspect(context)}
"""
|> IO.puts()

:ok
end
end
```

Start with the /var/task/bootstrap entry point and the hello_world_lambda.ex example, then reproduce the aws lambda invoke command while reviewing the logged startup commands. Trace which startup step attempts to create /var/task/var; done means the runtime can be invoked without the read-only filesystem error and the function reaches its normal response path.

Written by the indexing model from the issue text.