Ensure git clean state after running license_finder

What does this MR do and why?

Ensure git clean state after running license_finder

Our script for validating licenses does not restore a clean git state because the whole script scripts/license-check.sh is marked as set -e which means if any command fails with a non-zero exit code the script terminates its execution. The failing scenario is:

1. We start license-check.sh script which modifies Gemfile.lock due to a workaround described inside the script.
2. Then we run license_finder gem.
3. If it exits with a non-zero status code then the next command git checkout -q will never execute.

The simplest solution to resolve that failing scenario is to capture the exit code of license_finder gem, store it, restore the git clean state and exit with this exit code.

Please see this CI job for an example.

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