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/main_test.rb

@@ -97,7 +97,7 @@ class MainTest < IntegrationTest
 
   private
     def assert_local_env_file(contents)
-      assert_equal contents, deployer_exec("cat .env", capture: true)
+      assert_equal contents, deployer_exec("cat .kamal/env", capture: true)
     end
 
     def assert_envs(version:)
@@ -127,7 +127,7 @@ class MainTest < IntegrationTest
     end
 
     def remove_local_env_file
-      deployer_exec("rm .env")
+      deployer_exec("rm .kamal/env")
     end
 
     def assert_remote_env_file(contents, vm:)