[user-environment] Using $(hostname -s) vs ${HOSTNAME}

[cjac]

Recent change in the hostname implementation has required that we find short hostname using a different mechanism. This patch makes that change.

[cjac]

/gcbrun

[cjac]

no bazel tests for this action:

2024-02-23T19:50:00.272592904Z + bazel test --jobs=10 --local_test_jobs=10 --flaky_test_attempts=3 --action_env=INTERNAL_IP_SSH=true --test_output=errors --noshow_progress --noshow_loading_progress --test_arg=--image_version=1.5-ubuntu18 :test_user_environment
2024-02-23T19:50:00.318937577Z Extracting Bazel installation...
2024-02-23T19:50:08.775690454Z Starting local Bazel server and connecting to it...
2024-02-23T19:50:20.683601960Z WARNING: --enable_bzlmod is set, but no MODULE.bazel file was found at the workspace root. Bazel will create an empty MODULE.bazel file. Please consider migrating your external dependencies from WORKSPACE to MODULE.bazel. For more details, please refer to https://github.com/bazelbuild/bazel/issues/18958.
2024-02-23T19:50:35.614388397Z ERROR: Skipping ':test_user_environment': no such target '//:test_user_environment': target 'test_user_environment' not declared in package '' defined by /init-actions/BUILD (Tip: use `query "//:*"` to see all the targets in that package)
2024-02-23T19:50:35.662532758Z ERROR: no such target '//:test_user_environment': target 'test_user_environment' not declared in package '' defined by /init-actions/BUILD (Tip: use `query "//:*"` to see all the targets in that package)
2024-02-23T19:50:35.967725239Z INFO: Elapsed time: 35.219s
2024-02-23T19:50:35.969935227Z INFO: 0 processes.
2024-02-23T19:50:35.971904912Z ERROR: Build did NOT complete successfully
2024-02-23T19:50:35.979570415Z ERROR: Couldn't start the build. Unable to run tests

@functicons - we could either force push this change or maybe even remove the action. I've never received a customer case for it, and we could do with less code to maintain.

@medb - you've made changes to this action most recently. Do you think we can jettison it?