Respect cargo offline env variable in wasm builder (#11735)

* Support offline env variable in wasm builder

* Clean up

* Improve checks

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Update crate docs

* Add docs to `lib.rs` and introduce helper method `offline_build`

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/utils/wasm-builder/src/lib.rs

@@ -87,6 +87,8 @@
 //!   required as we walk up from the target directory until we find a `Cargo.toml`. If the target
 //!   directory is changed for the build, this environment variable can be used to point to the
 //!   actual workspace.
+//! - `CARGO_NET_OFFLINE` - If `true`, `--offline` will be passed to all processes launched to
+//!   prevent network access. Useful in offline environments.
 //!
 //! Each project can be skipped individually by using the environment variable
 //! `SKIP_PROJECT_NAME_WASM_BUILD`. Where `PROJECT_NAME` needs to be replaced by the name of the
@@ -119,6 +121,9 @@ pub use builder::{WasmBuilder, WasmBuilderSelectProject};
 /// Environment variable that tells us to skip building the wasm binary.
 const SKIP_BUILD_ENV: &str = "SKIP_WASM_BUILD";
 
+/// Environment variable that tells us whether we should avoid network requests
+const OFFLINE: &str = "CARGO_NET_OFFLINE";
+
 /// Environment variable to force a certain build type when building the wasm binary.
 /// Expects "debug", "release" or "production" as value.
 ///