Bug 1640429 - Run v8 fetch with --force. r=jmaher

This patch attempts to fix the long-running customv8 toolchain tasks by adding a --force to the `fetch v8` call. This additional option prevents `fetch` from checking for existing .gclient files (which should never exist) to help with the long delay we are seeing in this call.

Differential Revision: https://phabricator.services.mozilla.com/D76857

taskcluster/scripts/misc/build-custom-v8.sh

@@ -26,7 +26,7 @@ git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git
 export PATH=$PATH:$GECKO_PATH/depot_tools
 
 # Get v8 source code and dependencies
-fetch v8
+fetch --force v8
 cd v8
 
 # Build v8