ci: fix unquoted expression in images-dev shell condition

[immanuwell]

images-dev.yaml "Getting image tag" step runs this on push-to-main events:

if [ ${{ github.event.pull_request.head.sha }} != "" ]; then

github.event.pull_request.head.sha is empty on push events, so bash sees [ != "" ] and logs [: !=: unary operator expected on every merge to main. CI still passes (else branch runs fine), but the error shows up in every run.

Fix: quote the expression so bash sees [ "" != "" ] instead.

To reproduce: check the "Getting image tag" step in any Dev Image CI Build run triggered by a push to main.