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/xceadmin/meson.build

@@ -68,4 +68,12 @@ xceadmin_stamp = custom_target('xceadmin',
 # (ninja install always depends on build_by_default targets first) so the
 # build-dir venv it copies from already exists. See meson-install.sh for
 # why this isn't a plain `cp` of that venv.
-meson.add_install_script('meson-install.sh', python3_prog.full_path())
+# Invoke the install script through an explicit bash (find_program('bash'),
+# the same interpreter-pinning pattern the build-phase custom_target above
+# already uses) rather than relying on the script's `#!/usr/bin/env bash`
+# shebang. In the Nix install-phase sandbox there is no /usr/bin/env, so
+# letting meson exec the script by shebang fails with "could not be run";
+# handing meson a resolved bash + the script path sidesteps that entirely.
+meson.add_install_script(find_program('bash'),
+                         meson.current_source_dir() / 'meson-install.sh',
+                         python3_prog.full_path())