os: avoid allocation in File.WriteString

Instead of alloc+copy to convert the string
to a byte slice, do an unsafe conversion.

Rely on the kernel not to scribble on the
buffer during the write.

Fixes #42406

Change-Id: I66f4838b43a11bcc3d67bbfa1706726318d55343
Reviewed-on: https://go-review.googlesource.com/c/go/+/268020


Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>