Support masking multi-line values

[channingko-madden]

Hello!

My use case for wanting to mask the env variable values #2 involves multi-line values. I came across this article on masking multi-line text, and incorporated it into the yaml-to-env.sh script. Testing with my fork it is working for me and the multi-line values are all correctly masked.

I updated the Test Action workflow which should show it is working correctly as well. An example of the workflow running and variables being masked can be found here, under the Test env step.