path fetcher: Allow the lastModified attribute to be overriden again

[edolstra]

Motivation

It's common to have local registry overrides like

      "from": {
        "id": "nixpkgs",
        "type": "indirect"
      },
      "to": {
        "lastModified": 1728500571,
        "narHash": "sha256-dOymOQ3AfNI4Z337yEwHGohrVQb4yPODCW9MDUyAc4w=",
        "path": "/nix/store/m1szqwijms610n6325mwjswslha4nd92-source",
        "rev": "d51c28603def282a24fa034bcb007e2bcb5b5dd0",
        "revCount": 635766,
        "type": "path"
      }

However, this only worked by accident because the presence of the narHash attribute inhibited checking of attributes (see #10601, #10612). Now that we do check them, we have to explicitly respect the user-supplied lastModified.

Fixes #11660.

Context

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[bryanhonof]

LGTM!

question(non-blocking): Are there some sort of tests that already capture this behavior?

[bobvanderlinden]

This fixes the problem described in #11660 for me 👍

[edolstra]

@bryanhonof I've added a test.