build: remove v build file.v (#7695)

cmd/v/help/build.txt

@@ -1,4 +1,4 @@
-Usage: v [build flags] ['build'] <file.v|directory>
+Usage: v [build flags] <file.v|directory>
 
 This command compiles the given target, along with their dependencies, into an executable.
 

vlib/v/pref/pref.v

@@ -384,6 +384,10 @@ pub fn parse_args(args []string) (&Preferences, string) {
 				i++
 			}
 			else {
+				if command == 'build' && (arg.ends_with('.v') || os.exists(command)) {
+					eprintln('Use `v $arg` instead.')
+					exit(1)
+				}
 				if arg[0] == `-` {
 					if arg[1..] in list_of_flags_with_param {
 						// skip parameter