Skip cap_build example unless on Morello

CI was failing due to missing symbols, because it was compiling for
riscv but cap_build relies on morello-specific symbols.

cap_build.c

@@ -23,6 +23,11 @@
 
 int main()
 {
+    // This example relies on morello-specific symbols, so we won't run
+    // it on anything else.
+#if !(defined(__aarch64__) && __ARM_ARCH == 8)
+    printf("Skipping; this example only runs on morello.");
+#else
     // We create two capabilities: x, and y, which is derived from x. We note
     // that, though we use `cheri_address_set()`, it doesn't actually set (i.e.
     // _change_) the address of its arguments, but construct a new capability
@@ -150,4 +155,5 @@ int main()
     free((void *) y_ptr);
     free(my_cap_mem);
     return 0;
+#endif
 }