build: explicitly set SKIP_LABEL_TEST_FAILURE in compose.sh

Previously, `SKIP_LABEL_TEST_FAILURE` was being set via a teamcity
configuration. This change was quite opaque as the majority of CI
configuration for Cockroach is stored as shell scripts within its repo.
This commit follows that pattern by explicitly setting
`SKIP_LABEL_TEST_FAILURE` in the script that runs `TestComposeCompare`.

Epic: None
Release note: None

build/teamcity/cockroach/nightlies/compose.sh

@@ -6,6 +6,12 @@ dir="$(dirname $(dirname $(dirname $(dirname "${0}"))))"
 source "$dir/teamcity-support.sh"
 source "$dir/teamcity-bazel-support.sh"
 
+# The test failures generated by TestComposeCompare are not necessarily
+# failures per se. They're cases of behavioral divergences from Postgres. While
+# our compatibility guarantees are not 100%, it's better to treat failures as
+# information to occasionally review.
+export SKIP_LABEL_TEST_FAILURE=1
+
 tc_start_block "Run compose tests"
 
 bazel build //pkg/cmd/bazci --config=ci