buildbot: Archive successful builds.

Successful builds are tarred up and placed in /opt/ykrustc-bin-snapshots
on the build master.

For now, store at most one archive, but we might change that in the
future.

.buildbot.sh

@@ -2,9 +2,23 @@
 #
 # Build script for continuous integration.
 
+set -e
+
 # Ensure the build fails if it uses excessive amounts of memory.
 ulimit -d $((1024 * 1024 * 8)) # 8 GiB
 
 # Note that the gdb must be Python enabled.
 /usr/bin/time -v env PATH=/opt/gdb-8.2/bin:${PATH} \
     RUST_BACKTRACE=1 ./x.py test --config .buildbot.toml
+
+# Archive the build and put it in /opt
+TARBALL_TOPDIR=ykrustc-stage2
+TARBALL_NAME=ykrustc-stage2-latest.tar.bz2
+SNAP_DIR=/opt/ykrustc-bin-snapshots
+
+cd build/x86_64-unknown-linux-gnu
+ln -sf stage2 ${TARBALL_TOPDIR}
+git show -s HEAD > ${TARBALL_TOPDIR}/VERSION
+tar hjcvf ${TARBALL_NAME} ${TARBALL_TOPDIR}
+chmod 775 ${TARBALL_NAME}
+mv ${TARBALL_NAME} ${SNAP_DIR} # Overwrites any old archive.