Better error message if env var is used as var (#9522)

# Description

This PR improves the error message if an environment variable (that's
visible before the parser begins) is used in the form of `$PATH` instead
of `$env.PATH`.

Before:

```
Error: nu::parser::variable_not_found

  × Variable not found.
   ╭─[entry #31:1:1]
 1 │ echo $PATH
   ·      ──┬──
   ·        ╰── variable not found.
   ╰────
```

After:

```
Error: nu::parser::env_var_not_var

  × Use $env.PATH instead of $PATH.
   ╭─[entry #1:1:1]
 1 │ echo $PATH
   ·      ──┬──
   ·        ╰── use $env.PATH instead of $PATH
   ╰────
```

# User-Facing Changes

Just the improvement to the error message

# Tests + Formatting
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crates/nu-protocol/src/parse_error.rs

@@ -173,6 +173,10 @@ pub enum ParseError {
     #[diagnostic(code(nu::parser::variable_not_found))]
     VariableNotFound(DidYouMean, #[label = "variable not found. {0}"] Span),
 
+    #[error("Use $env.{0} instead of ${0}.")]
+    #[diagnostic(code(nu::parser::env_var_not_var))]
+    EnvVarNotVar(String, #[label = "use $env.{0} instead of ${0}"] Span),
+
     #[error("Variable name not supported.")]
     #[diagnostic(code(nu::parser::variable_not_valid))]
     VariableNotValid(#[label = "variable name can't contain spaces or quotes"] Span),
@@ -492,6 +496,7 @@ impl ParseError {
             ParseError::IncorrectValue(_, s, _) => *s,
             ParseError::MultipleRestParams(s) => *s,
             ParseError::VariableNotFound(_, s) => *s,
+            ParseError::EnvVarNotVar(_, s) => *s,
             ParseError::VariableNotValid(s) => *s,
             ParseError::AliasNotValid(s) => *s,
             ParseError::CommandDefNotValid(s) => *s,