Run ENV_CONVERSIONS whenever it's modified (#14591)

- this PR should close #14514

# Description
Makes updates to `$env.ENV_CONVERSIONS` take effect immediately.

# User-Facing Changes
No breaking change, `$env.ENV_CONVERSIONS` can be set and its effect
used in the same file.

# Tests + Formatting

- 🟢 toolkit fmt
- 🟢 toolkit clippy
- 🟢 toolkit test
- 🟢 toolkit test stdlib

# After Submitting
N/A

crates/nu-engine/src/eval_ir.rs

@@ -14,7 +14,9 @@ use nu_protocol::{
 };
 use nu_utils::IgnoreCaseExt;
 
-use crate::{eval::is_automatic_env_var, eval_block_with_early_return};
+use crate::{
+    convert_env_vars, eval::is_automatic_env_var, eval_block_with_early_return, ENV_CONVERSIONS,
+};
 
 /// Evaluate the compiled representation of a [`Block`].
 pub fn eval_ir_block<D: DebugContext>(
@@ -384,10 +386,16 @@ fn eval_instruction<D: DebugContext>(
 
             if !is_automatic_env_var(&key) {
                 let is_config = key == "config";
-                ctx.stack.add_env_var(key.into_owned(), value);
+                let update_conversions = key == ENV_CONVERSIONS;
+
+                ctx.stack.add_env_var(key.into_owned(), value.clone());
+
                 if is_config {
                     ctx.stack.update_config(ctx.engine_state)?;
                 }
+                if update_conversions {
+                    convert_env_vars(ctx.stack, ctx.engine_state, &value)?;
+                }
                 Ok(Continue)
             } else {
                 Err(ShellError::AutomaticEnvVarSetManually {