Allow overriding outputHashSalt in modifyConfig

[mischnic]

That hashSalt property copied the value from nextConfig.experimental.outputHashSalt too early. Values assigned by adapters via modifyConfig (which happens after assignDefaultsAndValidate) were ignored.

Instead handle it similarly to what we do for process.env.NEXT_DEPLOYMENT_ID as well

Fixup for #91871

[mischnic]

• Allow overriding outputHashSalt in modifyConfig #92856 👈 (View in Graphite)
• canary

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[vercel]

Notifying the following users due to files changed in this PR based on this repo's notify modifiers:

@timneutkens, @ijjk, @shuding, @huozhi:

packages/next/src/server/config.ts

[codspeed-hq]

Merging this PR will improve performance by 3.58%

⚡ 1 improved benchmark
✅ 16 untouched benchmarks
⏩ 3 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	app-page-turbo.runtime.prod.js[full]	640.9 ms	618.7 ms	+3.58%

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_{Comparing mischnic/salt-adapter (6def13d) with canary (bf4de72)}

Footnotes

1. 3 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩