libflake-tests: fix leak in nix_api_store_test.nix_api_load_flake_with_flags

[corngood]

Motivation

This test fails on cygwin:

[ RUN      ] nix_api_store_test.nix_api_load_flake_with_flags
warning: not writing modified lock file of flake 'path:/nix/var/nix/builds/nix-42403-950678037/nix-42408-3476222066/a':
• Added input 'b':
    'path:/nix/var/nix/builds/nix-42403-950678037/nix-42408-3476222066/b?lastModified=1767989117&narHash=sha256-nmA1vVxlRS1Y1wCpntvetQpa61NB/OvvroPCiZwRc%2BM%3D' (2026-01-09)
warning: creating lock file "/nix/var/nix/builds/nix-42403-950678037/nix-42408-3476222066/a/flake.lock":
• Added input 'b':
    'path:/nix/var/nix/builds/nix-42403-950678037/nix-42408-3476222066/b?lastModified=1767989117&narHash=sha256-nmA1vVxlRS1Y1wCpntvetQpa61NB/OvvroPCiZwRc%2BM%3D' (2026-01-09)
warning: not writing modified lock file of flake 'path:/nix/var/nix/builds/nix-42403-950678037/nix-42408-3476222066/a':
• Updated input 'b':
    'path:/nix/var/nix/builds/nix-42403-950678037/nix-42408-3476222066/b?lastModified=1767989117&narHash=sha256-nmA1vVxlRS1Y1wCpntvetQpa61NB/OvvroPCiZwRc%2BM%3D' (2026-01-09)
  → 'path:/nix/var/nix/builds/nix-42403-950678037/nix-42408-3476222066/c?lastModified=1767989117&narHash=sha256-7aeUAJM1XBrPeH/XiwlMaFnz3RPCqTgvlNSGEK8cR10%3D' (2026-01-09)
terminate called after throwing an instance of 'std::filesystem::__cxx11::filesystem_error'
  what():  filesystem error: cannot remove all: Device or resource busy [/nix/var/nix/builds/nix-42403-950678037/tests_nix-store.pUTLXB] [/nix/var/nix/builds/nix-42403-950678037/tests_nix-store.pUTLXB/my_state/db/db.sqlite]
/nix/var/nix/builds/nix-42403-950678037/.attr-0l2nkwhif96f51f4amnlf414lhl4rv9vh8iffyp431v6s28gsr90: line 3: 42408 Aborted                    /nix/store/hl337315ycb68z69mrhwwr8h0w8w8q6g-exec/bin/exec /nix/store/ppa17sj1x080dans4nhrwhxd22ic1lxa-nix-flake-tests-2.34pre20251217_b6add8dc+5/bin/nix-flake-tests.exe

Context

A fix for the problem described here may also be required for this test to actually pass.

---

Add 👍 to pull requests you find important.

The Nix maintainer team uses a GitHub project board to schedule and track reviews.

[Ericson2314]

@xokdvium should be using std::unique_ptr and RAII in these tests too? Seems like it would be a bit more robust.