roachtest: export $root for roachtest weekly

The `roachtest_compile_bits.sh` now require `$root` to be defined, see cockroachdb#124592
for details. This change adds the required exports to the
`roachtest_weekly_impl.sh` script.

Release Note: None
Epic: None

build/teamcity/cockroach/nightlies/roachtest_weekly_impl.sh

@@ -4,7 +4,10 @@ set -exuo pipefail
 
 dir="$(dirname $(dirname $(dirname $(dirname "${0}"))))"
 
+# N.B. export variables like `root` s.t. they can be used by scripts called below.
+set -a
 source "$dir/teamcity-support.sh"
+set +a
 
 if [[ ! -f ~/.ssh/id_rsa.pub ]]; then
   ssh-keygen -q -C "roachtest-weekly-bazel $(date)" -N "" -f ~/.ssh/id_rsa