tests: Support check-cfg for skip_gpu_tests. (#582)

crates/tests/build.rs

diff --git a/crates/tests/build.rs b/crates/tests/build.rs
index 6e81f57..3c08a0c 100644
--- a/crates/tests/build.rs
+++ b/crates/tests/build.rs
@@ -5,6 +5,7 @@
 
 fn main() {
     println!("cargo:rerun-if-env-changed=VELLO_CI_GPU_SUPPORT");
+    println!("cargo:rustc-check-cfg=cfg(skip_gpu_tests)");
     if let Ok(mut value) = env::var("VELLO_CI_GPU_SUPPORT") {
         value.make_ascii_lowercase();
         match &*value {