bazel_rules_test broken at HEAD

[c-parsons]

Description of the problem / feature request:

Bazel with downstream projects: "Bazel Remote Execution" is broken at HEAD.
Example run:
https://buildkite.com/bazel/bazel-with-downstream-projects-bazel/builds/341#b1274694-06b0-4b16-8ff9-d967073f0efe

Relevant snip:

** test_genrule_default_env ****************************************************
/b/s/w/ir/run/bazel-out/k8-fastbuild/bin/src/test/shell/bazel/bazel_rules_test.runfiles/io_bazel/src/test/shell/bazel/bazel_rules_test: line 295: TMPDIR: unbound variable

I believe this was caused by f814454

we probably just need to not assume TMPDIR is set, which was the previous state before that change

[laszlocsomor]

Taking a look.

[laszlocsomor]

we probably just need to not assume TMPDIR is set, which was the previous state before that change

$TMPDIR should always be set, see

bazel/src/main/java/com/google/devtools/build/lib/exec/local/PosixLocalEnvProvider.java

Line 55 in ae43043

result.put("TMPDIR", p);

However, it seems to be unset here. Let me take a look as to why.

[laszlocsomor]

Ah, because we're talking about remote execution, not local ones. I don't know how the remote executor sets up envvars, so let me add code that allows an empty TMPDIR.