Fix test_env to use build, fixing analysis caching

[jonmeow]

Per https://docs.bazel.build/versions/master/guide.html#option-defaults, test inherits from build, thus why this fixes caching.

The issue can be observed with bazel build :all && bazel test :all, the line:
INFO: Build option --test_env has changed, discarding analysis cache.

[tkoeppe]

Bazel is a bit extreme when it comes to cache invalidation. In principle you could use --trim_test_configuration to exclude lots of flags (which should only affect test runtime) from the build cache key, but I've seen odd cornercases where tests depend on the trimmed flags, so I no longer have that in my default .bazelrc...

[jonmeow]

Bazel is a bit extreme when it comes to cache invalidation. In principle you could use --trim_test_configuration to exclude lots of flags (which should only affect test runtime) from the build cache key, but I've seen odd cornercases where tests depend on the trimmed flags, so I no longer have that in my default .bazelrc...

--trim_test_configuration doesn't affect this issue (I checked to make sure), and will will be the default in future releases per bazelbuild/bazel#6842.

[chandlerc]

LGTM given our constraints...