feat: verify github url before updating (fixes #552)

[lnk3]

I will track progress in the PR from the start, so more experienced contributors can give feedback along the way.
I will squash and rebase at the end to have a clean history like it's done in nixpkgs.

Error to fix

$ cabal v2-run nixpkgs-update -- update "steam-tui 0.3.0 0.3.0b https://repology.org/project/steam-tui/versions"
Up to date
[options] github_user: r-ryantm, pull_request: NO, batch_update: NO, calculate_outpaths: NO, cve_report: NO, nixpkgs-review: NO, nixpkgs_dir: "/home/user/.cache/nixpkgs", use attrpath: NO
For attrpath steam-tui, using log file: /run/user/1000/nixpkgs-update/steam-tui/2026-06-17.log
steam-tui 0.3.0 -> 0.3.0b https://repology.org/project/steam-tui/versions
attrpath: steam-tui
[version]
[version] generic version rewriter does not support multiple hashes
[rustCrateVersion]
stderr did not split as expected full stderr was:
warning: unknown experimental feature 'pipe-operators'
warning: unknown setting 'always-allow-substitutes'
this derivation will be built:
  /nix/store/07am4w42dwck9xh0hdz5sh8x1kd41rj6-source.drv
building '/nix/store/07am4w42dwck9xh0hdz5sh8x1kd41rj6-source.drv'...
structuredAttrs is enabled

trying https://github.com/dmadisetti/steam-tui/archive/0.3.0b.tar.gz
...
curl: (22) The requested URL returned error: 404
error: cannot download source from any mirror
error: Cannot build '/nix/store/07am4w42dwck9xh0hdz5sh8x1kd41rj6-source.drv'.
...
error: Build failed due to failed dependency

[result] Failed to update steam-tui 0.3.0 -> 0.3.0b https://repology.org/project/steam-tui/versions

Error fixed

$ cabal v2-run nixpkgs-update -- update "steam-tui 0.3.0 0.3.0b https://repology.org/project/steam-tui/versions"
Up to date
[options] github_user: r-ryantm, pull_request: NO, batch_update: NO, calculate_outpaths: NO, cve_report: NO, nixpkgs-review: NO, nixpkgs_dir: "/home/user/.cache/nixpkgs", use attrpath: NO
For attrpath steam-tui, using log file: /run/user/1000/nixpkgs-update/steam-tui/2026-06-17.log
steam-tui 0.3.0 -> 0.3.0b https://repology.org/project/steam-tui/versions
attrpath: steam-tui
The new url doesn't exist: https://github.com/dmadisetti/steam-tui/archive/0.3.0b.tar.gz
[result] Failed to update steam-tui 0.3.0 -> 0.3.0b https://repology.org/project/steam-tui/versions

Tests

$ cabal v2-test
Build profile: -w ghc-9.2.4 -O1
In order, the following will be built (use -v for more details):
 - nixpkgs-update-0.4.0 (test:spec) (first run)
Preprocessing test suite 'spec' for nixpkgs-update-0.4.0..
Building test suite 'spec' for nixpkgs-update-0.4.0..
Running 1 test suites...
Test suite spec: RUNNING...
Test suite spec: PASS
Test suite logged to:
/home/user/nixpkgs-update/check-url-reachable-#552/dist-newstyle/build/x86_64-linux/ghc-9.2.4/nixpkgs-update-0.4.0/t/spec/test/nixpkgs-update-0.4.0-spec.log
1 of 1 test suites (1 of 1 test cases) passed.