refactor: fetch builds into bare mirrors

Host git only fetches into bare mirrors no sandbox ever sees, https only, with no user or system config and overrides emptying hooks, fsmonitor, ssh and proxy commands and every filter driver the mirror defines. Each build is a git archive of the commit and of each submodule commit from its own mirror. Old working-tree clones are deleted. The core copied out must be a regular file inside the build folder, opened without following links.

wiki/tools.md

@@ -955,7 +955,7 @@ python scripts/campaign.py install-timer     # the autorun units, the campaign's
 
 | Step | What happens |
 |------|--------------|
-| Core | The x86_64 Linux nightly from `buildbot.libretro.com`, for each name of the profile's `cores:` the buildbot publishes, downloaded again only when its `ETag` or `Last-Modified` changes. The record keeps the artifact URL, the `.so` sha256 and the `retro_get_system_info` version. With no nightly, the profile's `source_commit` is cloned and built once with `make` (`Makefile.libretro` first), the failure kept so it is not tried again for that commit. git clones with the allowlisted environment, no user or system config, and `core.hooksPath=/dev/null`, `core.fsmonitor=false` and the `file` and `ext` transports refused. The commit and its submodules are exported with `git archive` into a fresh folder, and `make` runs there in the same bubblewrap sandbox as a core, that folder the one writable path: the clone and its `.git` are never inside, so a build cannot plant a hook or a config line for the next host git command. The record's `build_command` says which isolation was used. A standalone emulator is `pending: standalone` |
+| Core | The x86_64 Linux nightly from `buildbot.libretro.com`, for each name of the profile's `cores:` the buildbot publishes, downloaded again only when its `ETag` or `Last-Modified` changes. The record keeps the artifact URL, the `.so` sha256 and the `retro_get_system_info` version. With no nightly, the profile's `source_commit` is cloned and built once with `make` (`Makefile.libretro` first), the failure kept so it is not tried again for that commit. Host git only fetches into bare mirrors (`.cache/campaign/mirrors/`), one per repository, submodules included, that it alone touches: no working tree, never bound into a sandbox, and earlier working-tree clones are deleted. It runs with no user or system config, https as the only transport, and `-c` overrides emptying hooks, `core.fsmonitor`, `core.sshCommand`, `core.gitProxy`, credential helpers and every filter driver the mirror's config defines. The commit and each submodule at the commit its tree records are exported with `git archive` into a fresh folder, and `make` runs there in the same bubblewrap sandbox as a core, that folder the one writable path. The core copied out is the newest `*_libretro.so` that is a regular file inside that folder, opened without following links. The record's `build_command` says which isolation was used. A standalone emulator is `pending: standalone` |
 | Content | Each system's games come from `tests/campaign/content.yml`, by catalog name and with the reason each was chosen. They are read from `.cache/test-content/<romfetch system>/<name>/`, each file checked against its `identity.json`. Missing content is fetched with `romfetch get <system> <name> --json` when romfetch is on `PATH`, in `RETROBIOS_ROMFETCH` or given with `--romfetch`. Otherwise the checks are `pending: content`, with romfetch's exit code and error code when it failed. Content never enters git, and a record keeps only its identity |
 | Checks | `files` stages every declared file the collection holds and reads which ones the core opens (strace). `boot` runs the game with no file and with the declared one (`boot_file:`, else the entry of the game's region). An entry the `files` run left unread is staged `alone`, then where the core looked for it (`path`) and under the names it looked for (`alias`). `variants` runs each `.variants/` copy of a file the core opened. `bench` times the game, warm-up excluded, 5 repetitions |
 | Rules | `campaign_rules.py`, a fixed table of pure functions (below). A field the table settles is written into the profile, with `evidence:` naming the record ids. Anything else is a line of `measurements/findings.jsonl` |