neon_local: background_process: launch all processes in repo dir (or datadir)

[problame]

Before this PR, storage controller and broker would run in the
PWD of neon_local, i.e., most likely the checkout of neon.git.

With this PR, the shared infrastructure for background processes
sets the PWD.

Benefits:

• easy listing of processes in a repo dir using lsof, see added
  comment in the code
• coredumps go in the right directory (next to the process)
• generally matching common expectations, I think

Changes:

• set the working directory in background_process module
• drive-by: fix reliance of storage_controller on NEON_REPO_DIR being set by neon_local for the local compute hook to work correctly

[github-actions]

3216 tests run: 3075 passed, 0 failed, 141 skipped (full report)

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Flaky tests (2)

Postgres 14

• test_subscriber_restart: release
• test_synthetic_size_while_deleting: release

Code coverage* (full report)

• functions: 31.5% (6636 of 21063 functions)
• lines: 48.6% (51635 of 106323 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
e14801e at 2024-06-14T13:51:19.322Z :recycle:}