cmd/buildlet: clamp modtime to be no greater than current time

Updates golang/go#19062

Change-Id: I12faf2941083d44466e6ff8a053c832631276ffe
Reviewed-on: https://go-review.googlesource.com/37381
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed