In steps/clean-acr-images, use dryRunArg to fit pipeline usage

[dagood]

• Fixes The steps/clean-acr-images.yml template always runs in dry run mode when called by stages/cleanup-acr-images.yml #2034

Seemed to work when temporarily applied onto the copied files for a Go cleanup pipeline. (Run)

[dagood]

Seemed to work

Confirmed that running the pipeline again detected and cleaned up no images.

[lbussell]

Confirmed that running the pipeline again detected and cleaned up no images.

@dagood Is cleaning up no images a good thing, in this instance? Or should it have cleaned up some images?

[dagood]

Ha, right--it's good. When we were unintentionally dry running, the cleanup pipeline reported that it (supposedly) cleaned up the same set of images every time, across a few runs. After this change, the first non-dry-run actually cleaned up images, so the second non-dry-run didn't see anything to clean up.

I'm inferring success based on that change in the logs because it's nontrivial for me to manually, directly check the prod resource to see if the cleanup actually happened. 😄

(The "--dry-run" arg also doesn't show up in our logs anymore, which is probably plenty of proof. But, the change in what the build saw in our ACR seems like a more end-to-end indicator.)