This is an adapted patch from the games/anki FreeBSD port.
https://github.com/freebsd/freebsd-ports/blob/109c3d4629b84972e660b689d169ac0761c1a519/games/anki/files/patch-build_configure_src_main.rs

It replaces or skips network-reliant methods of the build setup and disables
the setup for bundles and for offline documentation. This way, we achieve a
lean offline build.
Delete setup_python call to avoid setting an extra environment variable to
prevent network access.

The FreeBSD port offers greater flexibility with additional environment
variables such as OFFLINE_BUILD and should probably be upstreamed. I
have contacted the port maintainer (Kai Knoblich <kai@FreeBSD.org>).

From: Lucio Sauer <watermanpaint@posteo.net>
--- a/build/configure/src/main.rs
+++ b/build/configure/src/main.rs
@@ -11,25 +11,21 @@ mod web;
 
 use anyhow::Result;
 use aqt::build_and_check_aqt;
-use bundle::build_bundle;
 use ninja_gen::glob;
 use ninja_gen::inputs;
 use ninja_gen::protobuf::check_proto;
 use ninja_gen::protobuf::setup_protoc;
-use ninja_gen::python::setup_python;
 use ninja_gen::Build;
 use pylib::build_pylib;
 use pylib::check_pylib;
 use python::check_python;
-use python::setup_venv;
+use python::setup_venv_stub;
 use rust::build_rust;
 use rust::check_minilints;
 use rust::check_rust;
 use web::build_and_check_web;
 use web::check_sql;
 
-use crate::python::setup_sphix;
-
 fn anki_version() -> String {
     std::fs::read_to_string(".version")
         .unwrap()
@@ -44,16 +40,15 @@ fn main() -> Result<()> {
     setup_protoc(build)?;
     check_proto(build, inputs![glob!["proto/**/*.proto"]])?;
 
-    setup_python(build)?;
-    setup_venv(build)?;
+    println!("Patch: Using Python system environment.");
+    setup_venv_stub(build)?;
 
     build_rust(build)?;
     build_pylib(build)?;
     build_and_check_web(build)?;
     build_and_check_aqt(build)?;
-    build_bundle(build)?;
 
-    setup_sphix(build)?;
+    println!("Patch: Skipping build of bundles.");
 
     check_rust(build)?;
     check_pylib(build)?;