[llvm] workflows/build-ci-container: Fix typo in variable (PR #119943)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

I saw this with the new windows container job too. I think there are a couple other latent issues from splitting the build and push that need to be fixed though. Give me a little bit of time and I'll have a look.

https://github.com/llvm/llvm-project/pull/119943