testing: add a fake_crypto feature

eth2/utils/bls/build.rs

@@ -0,0 +1,19 @@
+// This build script is symlinked from each project that requires BLS's "fake crypto",
+// so that the `fake_crypto` feature of every sub-crate can be turned on by running
+// with FAKE_CRYPTO=1 from the top-level workspace.
+// At some point in the future it might be possible to do:
+// $ cargo test --all --release --features fake_crypto
+// but at the present time this doesn't work.
+// Related: https://github.com/rust-lang/cargo/issues/5364
+fn main() {
+    if let Ok(fake_crypto) = std::env::var("FAKE_CRYPTO") {
+        if fake_crypto == "1" {
+            println!("cargo:rustc-cfg=feature=\"fake_crypto\"");
+            println!("cargo:rerun-if-env-changed=FAKE_CRYPTO");
+            println!(
+                "cargo:warning=[{}]: Compiled with fake BLS cryptography. DO NOT USE, TESTING ONLY",
+                std::env::var("CARGO_PKG_NAME").unwrap()
+            );
+        }
+    }
+}