Disallow setting the PWD via load-env input (#12522)

# Description
Fixes #12520


# User-Facing Changes
Breaking change:

Any operation parsing input with `PWD` to set the environment will now
fail with `ShellError::AutomaticEnvVarSetManually`

Furthermore transactions containing the special env-vars will be
rejected before executing any modifications. Prevoiusly this was
changing valid variables before while leaving valid variables after the
violation untouched.

## `PWD` handling.

Now failing

```
{PWD: "/trolling"} | load-env
``` 

already failing 

```
load-env {PWD: "/trolling"}
``` 

## Error management



```
> load-env {MY_VAR1: foo, PWD: "/trolling", MY_VAR2: bar}
Error: nu:🐚:automatic_env_var_set_manually

  × PWD cannot be set manually.
   ╭─[entry #1:1:2]
 1 │  load-env {MY_VAR1: foo, PWD: "/trolling", MY_VAR2: bar}
   ·  ────┬───
   ·      ╰── cannot set 'PWD' manually
   ╰────
  help: The environment variable 'PWD' is set automatically by Nushell and cannot be set manually.
```

### Before:
```
> $env.MY_VAR1
foo
> $env.MY_VAR2
Error: nu:🐚:name_not_found
....
```
### After:
```
> $env.MY_VAR1
Error: nu:🐚:name_not_found
....
> $env.MY_VAR2
Error: nu:🐚:name_not_found
....
```

# After Submitting
We need to check if any integrations rely on this hack.

crates/nu-command/src/env/load_env.rs

@@ -1,4 +1,4 @@
-use nu_engine::{command_prelude::*, current_dir};
+use nu_engine::command_prelude::*;
 
 #[derive(Clone)]
 pub struct LoadEnv;
@@ -37,53 +37,34 @@ impl Command for LoadEnv {
         let arg: Option<Record> = call.opt(engine_state, stack, 0)?;
         let span = call.head;
 
-        match arg {
+        let record = match arg {
-            Some(record) => {
+            Some(record) => record,
-                for (env_var, rhs) in record {
-                    let env_var_ = env_var.as_str();
-                    if ["FILE_PWD", "CURRENT_FILE", "PWD"].contains(&env_var_) {
-                        return Err(ShellError::AutomaticEnvVarSetManually {
-                            envvar_name: env_var,
-                            span: call.head,
-                        });
-                    }
-                    stack.add_env_var(env_var, rhs);
-                }
-                Ok(PipelineData::empty())
-            }
             None => match input {
-                PipelineData::Value(Value::Record { val, .. }, ..) => {
+                PipelineData::Value(Value::Record { val, .. }, ..) => val.into_owned(),
-                    for (env_var, rhs) in val.into_owned() {
+                _ => {
-                        let env_var_ = env_var.as_str();
+                    return Err(ShellError::UnsupportedInput {
-                        if ["FILE_PWD", "CURRENT_FILE"].contains(&env_var_) {
-                            return Err(ShellError::AutomaticEnvVarSetManually {
-                                envvar_name: env_var,
-                                span: call.head,
-                            });
-                        }
-
-                        if env_var == "PWD" {
-                            let cwd = current_dir(engine_state, stack)?;
-                            let rhs = rhs.coerce_into_string()?;
-                            let rhs = nu_path::expand_path_with(rhs, cwd, true);
-                            stack.add_env_var(
-                                env_var,
-                                Value::string(rhs.to_string_lossy(), call.head),
-                            );
-                        } else {
-                            stack.add_env_var(env_var, rhs);
-                        }
-                    }
-                    Ok(PipelineData::empty())
-                }
-                _ => Err(ShellError::UnsupportedInput {
                         msg: "'load-env' expects a single record".into(),
                         input: "value originated from here".into(),
                         msg_span: span,
                         input_span: input.span().unwrap_or(span),
-                }),
+                    })
-            },
                 }
+            },
+        };
+
+        for prohibited in ["FILE_PWD", "CURRENT_FILE", "PWD"] {
+            if record.contains(prohibited) {
+                return Err(ShellError::AutomaticEnvVarSetManually {
+                    envvar_name: prohibited.to_string(),
+                    span: call.head,
+                });
+            }
+        }
+
+        for (env_var, rhs) in record {
+            stack.add_env_var(env_var, rhs);
+        }
+        Ok(PipelineData::empty())
     }
 
     fn examples(&self) -> Vec<Example> {