Avoid multi-line commands and fix exit code in package-and-qa job

What does this MR do and why?

Fixes the CI script for package-and-qa so that it captures the exit code for package_and_qa_check. Previously it had a null value:

Downstream jobs will not be triggered because package_and_qa_check exited with code: 
sh: 0: unknown operand
sh: 1: unknown operand

Now it captures the value correctly:

$ exit_code=0 && tooling/bin/qa/package_and_qa_check ${CHANGES_DIFFS_DIR} || exit_code=$?
$ echo $exit_code
2
$ if [ $exit_code -eq 0 ]; then # collapsed multi-line command
Downstream jobs will not be triggered because package_and_qa_check exited with code: 2

MR acceptance checklist

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