Replace .env* with .kamal/env*

By default look for the env file in .kamal/env to avoid clashes with
other tools using .env.

For now we'll still load .env and issue a deprecation warning, but in
future we'll stop reading those.

test/integration/docker/deployer/app/.kamal/env.erb

@@ -0,0 +1,2 @@
+SECRET_TOKEN='1234 with "中文"'
+SECRET_TAG='TAGME'