build: make Go and Python builds fully offline (hermetic Nix build)

The Nix build previously fetched Go modules from goproxy.cn and Python
build deps from PyPI during the build phase, relying on network leaking
into the sandbox. This is non-reproducible and breaks whenever the
sandbox resolver is unreachable (e.g. pc13v's loopback dnsmasq).

Go:
- Vendor all three modules (frontend/apiserver, frontend/xcecli,
  autoscaler/providers/baidu/src/bcctool) and commit vendor/ trees.
- tools/go_build now builds with GOFLAGS=-mod=vendor and GOPROXY=off
  instead of GOPROXY=https://goproxy.cn.

Python (xceadmin):
- Add setuptools + wheel to xce.nix nativeBuildInputs.
- meson-build.sh installs with pip --no-build-isolation --no-index
  --no-deps so no PyPI access happens at build time.

xceadmin install script:
- Invoke meson-install.sh via find_program('bash') instead of relying
  on its #!/usr/bin/env bash shebang, which fails in the Nix install
  sandbox (no /usr/bin/env -> 'could not be run').

Cleanup:
- Drop now-unneeded cacert and impureEnvVars=[HOME] from xce.nix and
  xcecli.nix (they only existed to enable the impure network fetch).

Docs:
- README build guide: drop '--option sandbox false' from nix build /
  nix bundle commands, note the build is now hermetic, and document the
  go mod vendor + git add vendor/ maintenance workflow.

Verified: nix build .#xcecore, .#xcecore_debug, .#xcecli, and .#xce all
succeed with the sandbox enabled.

src/nixSupport/xcecli.nix

@@ -17,14 +17,16 @@ stdenv.mkDerivation {
     meson
     ninja
     go
-    cacert
+    # cacert removed: Go build is now offline (vendored, GOPROXY=off) --
+    # see nixSupport/xce.nix and tools/go_build.
     which
     findutils
   ];
 
   build-system = [ pkgs.meson ];
 
-  impureEnvVars = [ "HOME" ];
+  # impureEnvVars = [ "HOME" ] removed: offline vendored Go build needs no
+  # host env; tools/go_build sets its own HOME. See nixSupport/xce.nix.
 
   # See xce.nix for why build_time is computed here (build phase) rather
   # than at flake-evaluation time.