net-mgmt/thanos: Fix BUILD_DATE syntax to fix build

• Change the syntax of BUILD_DATE after switching to GO_LDFLAGS as

port was using double quotes but Uses/go.mk has single quotes where it
interpolates GO_LDFLAGS, which requires use of different syntax.

• Also switch date format of BUILD_DATE to classic ISO 8601.

PR: 295384
Reported by: Chad Jacob Milios <milios@ccsys.com>
Approved by: db@, yuri@ (Mentors, implicit)
Fixes: 2af7cdf6fd59 * net-mgmt/thanos: improve port