scripts/build/.variables: Support SOURCE_DATE_EPOCH

Repeated builds of docker is currently unreproducible as the embedded
time does not support SOURCE_DATE_EPOCH. This patch ensures we check for
the defined variable before utilizing current date.

This has been fixed previously with manpages as well

https://github.com/docker/cli/commit/2d7091aaeb3dac6388d81cbb9b6847c75ee4b7b9

Signed-off-by: Morten Linderud <morten@linderud.pw>

scripts/build/.variables

@@ -4,7 +4,7 @@ set -eu
 PLATFORM=${PLATFORM:-}
 VERSION=${VERSION:-"unknown-version"}
 GITCOMMIT=${GITCOMMIT:-$(git rev-parse --short HEAD 2> /dev/null || true)}
-BUILDTIME=${BUILDTIME:-$(date -u +"%Y-%m-%dT%H:%M:%SZ")}
+BUILDTIME=${BUILDTIME:-$(date -u --date="@${SOURCE_DATE_EPOCH:-$(date +%s)}" +"%Y-%m-%dT%H:%M:%SZ")}
 
 PLATFORM_LDFLAGS=
 if test -n "${PLATFORM}"; then