config: exclude DeprecatedWarnSetting names from GlobalConfig::toKeyValue()

[NorfairKing]

Fixes #15819.

Since nix 2.34 (commit 5184f844b), every nix command run inside a post-build-hook emits a spurious deprecation warning:

warning: 'warn-short-path-literals' is deprecated, use 'lint-short-path-literals = ignore' instead

even when the user never set warn-short-path-literals. The cause: DeprecatedWarnSetting registers the deprecated name via addSetting() with isAlias = false, making it a primary setting. Config::getSettings() skips aliases but includes primary settings. GlobalConfig::toKeyValue() calls getSettings() and serialises the result into NIX_CONFIG. The daemon injects this into every post-build-hook subprocess environment (derivation-goal.cc) and into nix repl subprocesses (repl.cc). Any nix command in the hook reads NIX_CONFIG, hits DeprecatedWarnSetting::assign(), and fires the warning.

Fix: add an excludeFromKeyValue flag to Config::SettingData. When set, getSettings() skips the setting, removing it from GlobalConfig::toKeyValue() / NIX_CONFIG. A new Config::excludeSettingFromKeyValue() method (which asserts the name exists, to catch misuse) sets the flag. DeprecatedWarnSetting calls it immediately after addSetting().

This is preferable to making the setting a true alias (isAlias = true), which would also drop the --warn-short-path-literals / --no-warn-short-path-literals CLI flags since convertToArgs() skips aliases. With excludeFromKeyValue, reading the setting from nix.conf / NIX_CONFIG still works and still emits the deprecation warning when explicitly set by the user; CLI flags are unaffected.

A regression test in tests/functional/short-path-literals.sh builds a derivation with a post-build-hook that captures $NIX_CONFIG and asserts warn-short-path-literals is absent.

Note: Root cause analysis and fix were produced with the help of an AI assistant (Claude), reviewed and confirmed against the source code by the author.