zig build --fetch doesn't fetch transitive dependencies

[mitchellh]

Zig Version

0.13.0

Steps to Reproduce and Observed Behavior

All maintainers of Zig should have access to Ghostty, so this reproduction uses Ghostty source:

With https://github.com/ghostty-org/ghostty/commit/59e226154c413a03b41b37ed8ec6c61193b340bd:

$ rm -rfv ~/.cache/zig
$ zig build --fetch # INTERNET ENABLED HERE
$ zig build # DISABLED INTERNET FOR THIS STEP
/home/x/.cache/zig/p/12207f53d7dddd3e5ca6577fcdd137dcf1fa32c9f22cbb0911ad0701cde4095a1c4c/build.zig.zon:7:20: error: unable to discover remote git server capabilities: TemporaryNameServerFailure
            .url = "git+https://github.com/zigimg/zigimg#3a667bdb3d7f0955a5a51c8468eac83210c1439e",
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/x/.cache/zig/p/12207f53d7dddd3e5ca6577fcdd137dcf1fa32c9f22cbb0911ad0701cde4095a1c4c/build.zig.zon:20:20: error: unable to discover remote git server capabilities: TemporaryNameServerFailure
            .url = "git+https://github.com/mitchellh/libxev#f6a672a78436d8efee1aa847a43a900ad773618b",
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If you check out an earlier commit https://github.com/ghostty-org/ghostty/commit/8211e7e83af26406fd451afef03b9c6e2bb5017d this works fine. The difference is that the commit that broke it introduces a dependency with transitive dependencies.

This use case is important to enable sandboxed builds in Nix.

Expected Behavior

This should work and allow builds without internet access.

[rohlem]

Related to #20716, although that one doesn't mention the --fetch flag - not sure whether the behavior should be different.