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

tests/shell/environment/env.rs

@@ -190,6 +190,16 @@ fn env_var_case_insensitive() {
     assert!(actual.out.contains("222"));
 }
 
+#[test]
+fn env_conversion_on_assignment() {
+    let actual = nu!(r#"
+        $env.FOO = "bar:baz:quox"
+        $env.ENV_CONVERSIONS = { FOO: { from_string: {|| split row ":"} } }
+        $env.FOO | to nuon
+    "#);
+    assert_eq!(actual.out, "[bar, baz, quox]");
+}
+
 #[test]
 fn std_log_env_vars_are_not_overridden() {
     let actual = nu_with_std!(