hplip: fix plugin fetch

[psentee]

HP now returns 403 for unrecognized user-agents:

building '/nix/store/r9airhak8ic5pqpmbl7v9ixv1sif43rn-hplip-3.24.4-plugin.run.drv'...
error: builder for '/nix/store/r9airhak8ic5pqpmbl7v9ixv1sif43rn-hplip-3.24.4-plugin.run.drv' failed with exit code 1;
       last 19 log lines:
       >
       > trying https://developers.hp.com/sites/default/files/hplip-3.24.4-plugin.run
       >   % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
       >                                  Dload  Upload   Total   Spent    Left  Speed
       >   0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
       > curl: (22) The requested URL returned error: 403
       > Warning: Problem (retrying all errors). Will retry in 1 seconds. 3 retries
       > Warning: left.
       >   0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
       > curl: (22) The requested URL returned error: 403
       > Warning: Problem (retrying all errors). Will retry in 2 seconds. 2 retries
       > Warning: left.
       >   0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
       > curl: (22) The requested URL returned error: 403
       > Warning: Problem (retrying all errors). Will retry in 4 seconds. 1 retries
       > Warning: left.
       >   0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
       > curl: (22) The requested URL returned error: 403
       > error: cannot download hplip-3.24.4-plugin.run from any mirror
       For full logs, run 'nix log /nix/store/r9airhak8ic5pqpmbl7v9ixv1sif43rn-hplip-3.24.4-plugin.run.drv'.

This change sets fetch user-agent to Firefox prefix (bare 'Mozilla/5.0' wasn't enough).

Also adds plugin source as a second source instead of a closed-over binding so it's possible to read and override it from outside (e.g. point at internal mirror or vendor the file).

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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