Exclude PPC64 from stable CI (requires nightly)

Author: walruscraft

.github/workflows/main.yml

@@ -24,6 +24,11 @@ jobs:
       matrix:
         build: [ubuntu, i686-linux, aarch64-linux, riscv64-linux, ppc64le-linux]
         rust: [1.88, nightly-2025-10-23]
+        exclude:
+          # PPC64 requires nightly for asm_experimental_arch; on stable,
+          # rustix uses libc backend which lacks the runtime module.
+          - build: ppc64le-linux
+            rust: 1.88
         include:
           - build: ubuntu
             os: ubuntu-latest