Fix Concourse builds by specifying Node minor version

[svenaas]

Concourse builds on #4145 were failing with

node[1]: ../src/node_platform.cc:68:std::unique_ptr<long unsigned int> node::WorkerThreadsTaskRunner::DelayedTaskScheduler::Start(): Assertion `(0) == (uv_thread_create(t.get(), start_thread, this))' failed.

but were resolved by specifying Node image tag 18.15 instead of 18.

This may be the underlying issue, as it evidently rears up with .Ubuntu 22.04 (credit to @drewbo for finding this).

Acceptance Criteria

• [x] Node 18.15 specified
• [x] Concourse build succeeds

Security Considerations

This change specifies a more specific minor version consistent with what was previously being used for builds. Instead of getting a newer minor version of Node 18 we are pinned to a specific minor version, which is an important detail, but not a problem at this time.

[drewbo]

Closed via https://github.com/cloud-gov/pages-core/pull/4151