Check for wrong or missing WORKING_DIR

Sven Hartgerequested to merge
check-working-dir into master

As pipeline#28 (closed) has shown, there are custom gitlab-ci.yml out there which haven't been updated to follow the latest version of salsa-ci.yml.

This commit updates the build script to check for one known error and aborts the build before we waste precious time just to fail at the end regardless.

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