[hf] inject p8s port as METRICS_PORT env var to signald

context

• in #326 (closed) , we want to run multiple instances of signald, each of which must run a prometheus server on a different port
• i preparation for that MR, we want to start using a build of signald that reads the prometheus server port from an injected env var (so that we can inject arbitrary values once we have many signald instances)

changes

use this build of signald:

• MR: signald-fork!23 (merged)
• commit: signald-fork@6cf17ecb
• which runs prometheus server on a port read from METRICS_PORT env var

inject METRICS_PORT into all signald isntances