Fix non-reproducible Linux kernel build due to timestamps

Linux's KBUILD doesn't follow SOURCE_DATE_EPOCH, but rather it
uses its own variable, KBUILD_BUILD_TIMESTAMP.

While we are at it, also update kexec-linux's checksum, which
didn't match either. (This one was reproducible, it was just
out of date.)

steps/improve/update_env.sh

@@ -16,6 +16,7 @@ SRCDIR=${SRCDIR}
 MAKEJOBS=-j${JOBS}
 export HOME=/tmp
 export SOURCE_DATE_EPOCH=0
+export KBUILD_BUILD_TIMESTAMP='@0'
 export SHELL=/usr/bin/bash
 DESTDIR=/tmp/destdir
 EOF