fix std log (#12470)

related to
- https://github.com/nushell/nushell/pull/12196

# Description
while i'm 100% okey with the original intent behind
https://github.com/nushell/nushell/pull/12196, i think the PR did
introduce two unintended things:
- extra parentheses that make the `log.nu` module look like Lisp lol
- a renaming of the `NU_LOG_LEVEL` environment variable to
`NU_log-level`. this breaks previous usage of `std log` and, as it's not
mentionned at all in the PR, i thought it was not intentional 😋

# User-Facing Changes
users can now control `std log` with `$env.NU_LOG_LEVEL`

# Tests + Formatting
the "log" tests have been fixed as well.

# After Submitting

crates/nu-std/tests/logger_tests/test_basic_commands.nu

@@ -6,9 +6,9 @@ def run [
     --short
 ] {
     if $short {
-        ^$nu.current-exe --no-config-file --commands $'use std; NU_log-level=($system_level) std log ($message_level) --short "test message"'
+        ^$nu.current-exe --no-config-file --commands $'use std; NU_LOG_LEVEL=($system_level) std log ($message_level) --short "test message"'
     } else {
-        ^$nu.current-exe --no-config-file --commands $'use std; NU_log-level=($system_level) std log ($message_level) "test message"'
+        ^$nu.current-exe --no-config-file --commands $'use std; NU_LOG_LEVEL=($system_level) std log ($message_level) "test message"'
     }
     | complete | get --ignore-errors stderr
 }