Bug 1394593 - Heroku: Use .profile instead of set-env

set-env was renamed in the buildpack to set_env, breaking this script.
Rather than just renaming our usage, it makes more sense to stop using
what is really an internal buildpack API and instead use `.profile`:
https://devcenter.heroku.com/articles/dynos#the-profile-file

bin/post_compile

@@ -14,19 +14,15 @@ yarn build
 # collectstatic, and so does not affect files in the `dist/` directory.
 python -m whitenoise.compress dist
 
-# The post_compile script is run in a sub-shell, so we need to source the
-# buildpack's utils script again, so we can use set-env/set-default-env:
-# https://github.com/heroku/heroku-buildpack-python/blob/master/bin/utils
-source $BIN_DIR/utils
-
 # Add environment variables to the profile script run on Dyno startup.
+# https://devcenter.heroku.com/articles/dynos#the-profile-file
 # Only variables that are rarely changed and not site-specific should
 # be set here. Set everything else using Heroku's CLI / dashboard.
 
-# Override the hostname that is displayed in New Relic, so the Dyno name
-# (eg "web.1") is shown, rather than "Dynamic Hostname". $DYNO is quoted
-# so that it's expanded at runtime on each dyno, rather than now.
-set-env NEW_RELIC_PROCESS_HOST_DISPLAY_NAME '$DYNO'
+# Override the hostname displayed in New Relic, so the Dyno name (eg "web.1")
+# is shown, rather than "Dynamic Hostname". Single quotes are used to so that
+# `$DYNO` is expanded at app runtime, rather than now.
+echo 'export NEW_RELIC_PROCESS_HOST_DISPLAY_NAME="${DYNO}"' >> .profile
 
 # Remove nodejs files to reduce slug size (and avoid environment variable
 # pollution from the nodejs profile script), since they are no longer