hack: Drop /tmp mounts

Eric points out potential issues with relabeling /tmp [1], which is
shared by several system-level consumers.

For the Bazel script, the /tmp mount is just since c483f59 (Move
bazel build tarball test to prow, 2018-08-08, #117), so we can drop it
to return to our previous approach.

The Terraform container seems to run fine without /tmp as well,
although there's no clear history to point to on this front because we
used to use Bazel for this.  See b8a9bbc (Remove bazel from test
process, 2018-08-01, #97).

[1]: #174 (comment)

hack/test-bazel-build-tarball.sh

@@ -6,7 +6,6 @@ else
   docker run --rm \
     --env IS_CONTAINER=TRUE \
     --volume "${PWD}:${PWD}:z" \
-    --volume /tmp:/tmp:z \
     --workdir "${PWD}" \
     quay.io/coreos/tectonic-builder:bazel-v0.3 \
     ./hack/test-bazel-build-tarball.sh

hack/tf-fmt.sh

@@ -8,7 +8,6 @@ else
   docker run --rm \
     --env IS_CONTAINER=TRUE \
     --volume "${PWD}:${PWD}:ro,z" \
-    --volume /tmp:/tmp:z \
     --workdir "${PWD}" \
     quay.io/coreos/terraform-alpine:v0.11.7 \
     ./hack/tf-fmt.sh