fix(test,doc): port env name change

Older JuliaHub jobimage environments used `PORT` as the environment variable name.
This was updated to `JULIAHUB_APP_PORT` in newer (v2) jobimage environment.

This updates the tests to check both. Also updates the example in the docs.

Author: tanmaykm

docs/src/guides/jobs.md

@@ -253,7 +253,9 @@ import JuliaHub # hide
 job = JuliaHub.submit_job(
     JuliaHub.script"""
     using Oxygen, HTTP
-    PORT = parse(Int, ENV["PORT"])
+    # Environment variable name `PORT` was used in older JuliaHub environments
+    # and has been replaced with `JULIAHUB_APP_PORT` in newer environments 
+    PORT = parse(Int, get(ENV, "JULIAHUB_APP_PORT", ENV["PORT"]))
     @get "/" function(req::HTTP.Request)
         return "success"
     end

test/jobenvs/job-exposed-port/server.jl

@@ -1,6 +1,8 @@
 using Oxygen, HTTP
 
-const PORT = parse(Int, ENV["PORT"])
+# Environment variable name `PORT` was used in older JuliaHub environments
+# and has been replaced with `JULIAHUB_APP_PORT` in newer environments 
+const PORT = parse(Int, get(ENV, "JULIAHUB_APP_PORT", ENV["PORT"]))
 const NREQUESTS = Ref{Int}(0)
 
 function results_json()