Use parameters instead of variables for pipeline variables that are intended to be manually set at queue time

We often need to set several pipeline variables when manually queuing some pipelines. For example, `imageBuilder.pathArgs` to specify which images to build. We set these variables manually at the pipeline level. AzDo pipelines has support for "Runtime Parameters" which gives a better UI for setting runtime variables and allows for the pipeline to expose which variables should be set instead of needing to set them manually via the pipeline UI.

Where possible, when working on https://github.com/dotnet/dotnet-docker-internal/issues/4475, we should try to use parameters where we were previously using variables (time permitting).