fix: supply the ninth format argument in build_guest_script

[bdelanghe]

This crate does not compile on main.

error: 9 positional arguments in format string, but there are 8 arguments
  --> src/script.rs:72:19

The guest script's format string has nine placeholders; the argument list supplies eight.

The missing argument is determined, not guessed

The missing one is a third bootstrap_path, at the fallback warning "bootstrap flake not found at {}".

The else branch reads: report that the local flake was missing, then retry via GitHub. So the warning takes the path and the nix develop that follows takes the URL — which is exactly where the argument list skipped from bootstrap_path straight to bootstrap_github.

Adds the tests that would have caught it

Nothing exercised build_guest_script at all — it's called only from lima.rs at runtime — which is how an outright compile error survived on main.

The second test earns its place. An arity mismatch is caught by the compiler, but swapping the path and URL arguments still compiles, and produces a script that names the GitHub URL in the "not found at" message and then runs the local path as a flake reference. I verified the test has teeth by making that swap and watching it fail:

test script::tests::bootstrap_fallback_reports_path_and_runs_github_url ... FAILED

then reverting.

Verification

• cargo build — clean
• cargo test — 18 passed, 0 failed (16 pre-existing + 2 new)

Relationship to the osv work

Unrelated to the dependency changes in the merged #4, though that's how it surfaced: the broken build made it impossible to confirm the git2 0.20 → 0.21 bump compiled, so that advisory fix landed on weaker evidence than it should have. With this merged, that bump is verifiable.

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