chromium_libfuzzer_clang_tot: Set LLVM_FORCE_HEAD_REVISION during runhooks.

scripts/slave/recipes/chromium_libfuzzer_clang_tot.expected/full_chromium_fyi_ClangToTLinuxASanLibfuzzer.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "chromium.fyi",
       "--builder",
       "ClangToTLinuxASanLibfuzzer",
@@ skipping 98 matching lines @@
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "runhooks"
     ],
     "cwd": "[SLAVE_BUILD]/src",
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json",
       "GYP_CHROMIUM_NO_ACTION": "1",
       "GYP_DEFINES": "clang=1 component=static_library gomadir='[BUILD]/goma' proprietary_codecs=1 target_arch=x64 use_goma=1",
+      "LLVM_FORCE_HEAD_REVISION": "YES",
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/src/tools/mb/mb.py",
       "gen",
@@ skipping 77 matching lines @@
       "-runs=1"
     ],
     "name": "running empty_fuzzer"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]