Check as-if-std only on nightly

Author: clubby789

.github/workflows/main.yml

@@ -137,8 +137,11 @@ jobs:
 
       # Test that including as a submodule will still work, both with and without
       # the `backtrace` feature enabled.
+      # Building as if part of std requires nightly features to be available
       - run: cargo build --manifest-path crates/as-if-std/Cargo.toml
+        if: matrix.rust == 'nightly'
       - run: cargo build --manifest-path crates/as-if-std/Cargo.toml --no-default-features
+        if: matrix.rust == 'nightly'
 
   windows_arm64:
     name: Windows AArch64