fix: respect pre-existing .env files when preparing the herokuish buildenv

Closes #4894

tests/apps/python/.env

@@ -0,0 +1 @@
+DOTENV_KEY=some_value

tests/apps/python/release.py

@@ -5,7 +5,10 @@ import os
 def main():
     print("GLOBAL_SECRET: {0}".format(os.getenv('GLOBAL_SECRET')))
     print("SECRET_KEY: {0}".format(os.getenv('SECRET_KEY')))
-
+    with open("/app/.env", "r") as f:
+        for line in f.readlines():
+            if "DOTENV_KEY" in line:
+                print(line)
 
 if __name__ == '__main__':
     main()