feat: serve the one-line installer from the site

The site serves install.sh at /retrobios/i and install.ps1 at
/retrobios/i.ps1. generate_site.py writes both copies with the
installer address set to install.py at the commit the site is built
from (GITHUB_SHA in CI, HEAD locally), so the pinned SHA-256 and the
downloaded installer always come from one commit, including while a
deploy is still running.

Every command a user copies names the short address. The raw form
stays on the installer page as the fallback. Deploy Site now runs on
changes to install.sh and install.ps1. The install box weight keeps
the address length its ratio was fitted on, so no console page moves.

scripts/validate_site.py

@@ -404,6 +404,7 @@ def _check_get_pages(
     """
     sys.path.insert(0, str(Path(__file__).resolve().parent))
     import release_record  # noqa: PLC0415
+    from siteguides import INSTALL_SH  # noqa: PLC0415
     from common import list_registered_platforms, list_release_pack_platforms  # noqa: PLC0415
 
     if not (platforms_dir / "_registry.yml").is_file():
@@ -428,7 +429,7 @@ def _check_get_pages(
             packs = [a for a in assets if "_BIOS_Pack" in a]
             if packs:
                 issues.append(f"{rel}: links a pack for a setup without one: {packs[0]}")
-            if "install.sh" not in (site / rel).read_text(encoding="utf-8"):
+            if f"{INSTALL_SH} | sh" not in (site / rel).read_text(encoding="utf-8"):
                 issues.append(f"{rel}: gives no install command")
     return issues