Refactor Crystal::System.retry_with_buffer calling #to_slice (#14614

)

This is a trivial refactor, moving the call to `StaticArray#to_slice` out of the loop. On repeat iterations `buf` is a `Slice` so `#to_slice` is actually a no-op.

src/crystal/system/unix.cr

@@ -2,9 +2,9 @@
 module Crystal::System
   def self.retry_with_buffer(function_name, max_buffer, &)
     initial_buf = uninitialized UInt8[1024]
-    buf = initial_buf
+    buf = initial_buf.to_slice
 
-    while (ret = yield buf.to_slice) != 0
+    while (ret = yield buf) != 0
       case ret
       when LibC::ENOENT, LibC::ESRCH, LibC::EBADF, LibC::EPERM
         return nil