cmd/go/internal: use time.DateTime constant
Use the newly defined time.Datetime constant instead of a string literal.
Updates #52746
Change-Id: I2722415ecc67fd2adfdab2eaba3298774032bff3
GitHub-Last-Rev:
65d3aa9bc229cf6d07d6d0c1aaee4ada9056cd49
GitHub-Pull-Request: golang/go#55833
Reviewed-on: https://go-review.googlesource.com/c/go/+/433277
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>