Fix Ruby version command

[ofalvai]

Checklist

• I've read and followed the Contribution Guidelines
• step.yml and README.md is updated with the changes (if needed)

Version

Requires a MAJOR/MINOR/PATCH version update

Context

Ruby version reporting got broken in the last commit of #95:

Determine desired Fastlane version
Gem lockfile does not exist
Failed to check active Ruby version: executing command failed (ruby "--version"): exec: Stdout already set
Output:

Changes

Fix the command execution params. We don't need to set the stdout, it's enough to set the workdir.

Investigation details

Decisions