change LOG_FORMAT to NU_LOG_FORMAT in nu-std library (#10254)

this closes #10248 

@fdncred pointed out the problem and he was correct 😄 

I went ahead and made the simple change and the
https://github.com/influxdata/influxdb_iox binary
works like a charm...

@amtoine hopefully there are no issues with this change

I believe its a good one as other rust binaries might take advantage of
this common
environment variable as well...

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

@@ -38,5 +38,5 @@ def env_log_short_prefix [] {
 
 #[test]
 def env_log_format [] {
-    assert equal $env.LOG_FORMAT $"%ANSI_START%%DATE%|%LEVEL%|(ansi u)%MSG%%ANSI_STOP%"
+    assert equal $env.NU_LOG_FORMAT $"%ANSI_START%%DATE%|%LEVEL%|(ansi u)%MSG%%ANSI_STOP%"
 }