Switch Ubuntu 14.04 step back to ubuntu1404 (bazelbuild#664)

It accidentally used ubuntu1804, which means we built a version no longer compatible with Ubuntu 14.04: bazelbuild/bazel#7499 (comment)

Note that Bazel 0.26 will be the last version built on Ubuntu 14.04, though, as was previously announced here: https://groups.google.com/forum/#!searchin/bazel-discuss/ubuntu$2014.04%7Csort:date/bazel-discuss/dPomOEOPseA/Jp849gJvBwAJ

pipelines/bazel-release.yml

@@ -42,7 +42,7 @@ steps:
           - ANDROID_HOME
           - ANDROID_NDK_HOME
           - BUILDKITE_ARTIFACT_UPLOAD_DESTINATION
-        image: gcr.io/bazel-public/ubuntu1804:java11
+        image: gcr.io/bazel-public/ubuntu1404:java8
         network: host
         privileged: true
         propagate-environment: true
@@ -140,7 +140,7 @@ steps:
           - ANDROID_HOME
           - ANDROID_NDK_HOME
           - BUILDKITE_ARTIFACT_UPLOAD_DESTINATION
-        image: gcr.io/bazel-public/ubuntu1804:java11
+        image: gcr.io/bazel-public/ubuntu1404:java8
         network: host
         privileged: true
         propagate-environment: true