[Backport release-2.16] fix: variable expansion on buildargs that only have one env

[github-actions]

Backport 0727f25 from #3689.

[jLopezbarb]

Needed a commit because #3635 is missing in the 2.16 branch

[derek]

Thank you for your contribution. unfortunately, one or more of your commits are missing the required "Signed-off-by:" statement. Signing off is part of the Developer Certificate of Origin (DCO) which is used by this project.

Read the DCO and project contributing guide carefully, and amend your commits using the git CLI. Note that this does not require any cryptography, keys or special steps to be taken.

💡 Shall we fix this?

This will only take a few moments.

First, clone your fork and checkout this branch using the git CLI.

Next, set up your real name and email address:

git config --global user.name "Your Full Name"
git config --global user.email "you@domain.com"

Finally, run one of these commands to add the "Signed-off-by" line to your commits.

If you only have one commit so far then run: git commit --amend --signoff and then git push --force.
If you have multiple commits, watch this video.

Check that the message has been added properly by running "git log".