chromiumBeta,chromiumDev: Mark as broken

The builds currently fail with (should work with LLVM 12 [0]):
../../base/check.h:88:3: error: 'nomerge' attribute cannot be applied to a declaration
  NOMERGE ~CheckError();
  ^       ~
../../base/compiler_specific.h:344:19: note: expanded from macro 'NOMERGE'
#define NOMERGE [[clang::nomerge]]
                  ^
1 error generated.

[0]: fb0f728805

pkgs/applications/networking/browsers/chromium/browser.nix

@@ -89,5 +89,6 @@ mkChromiumDerivation (base: rec {
       then ["aarch64-linux" "x86_64-linux"]
       else [];
     timeout = 172800; # 48 hours (increased from the Hydra default of 10h)
+    broken = elem channel [ "beta" "dev" ]; # Build requires LLVM 12
   };
 })