cleanup coreutils tagging (#12286)

# Description

Cleanup search terms and help usage to be consistent and include
coreutils so people can easily find out which commands are coreutils.


![image](https://github.com/nushell/nushell/assets/343840/09b03b11-19ce-49ec-b0b5-9b8455d1b676)

or
```nushell
help commands | where usage =~ coreutils | reject params input_output
```

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crates/nu-command/src/filesystem/mktemp.rs

@@ -21,12 +21,12 @@ impl Command for Mktemp {
 
     fn search_terms(&self) -> Vec<&str> {
         vec![
-            "coreutils",
             "create",
             "directory",
             "file",
             "folder",
             "temporary",
+            "coreutils",
         ]
     }
 

crates/nu-command/src/filesystem/umv.rs

@@ -43,7 +43,7 @@ impl Command for UMv {
     }
 
     fn search_terms(&self) -> Vec<&str> {
-        vec!["move"]
+        vec!["move", "file", "files", "coreutils"]
     }
 
     fn signature(&self) -> nu_protocol::Signature {

crates/nu-command/src/system/uname.rs

@@ -21,12 +21,12 @@ impl Command for UName {
     }
 
     fn usage(&self) -> &str {
-        "Print certain system information."
+        "Print certain system information using uutils/coreutils uname."
     }
 
     fn search_terms(&self) -> Vec<&str> {
         // add other terms?
-        vec!["system"]
+        vec!["system", "coreutils"]
     }
 
     fn run(