devops(ff-beta): fix build to use bootstrapped toolchains (#14491)

This is a follow-up to a7a7644beb
that started bootstrapping toolchains from `master` branch.

Now, we have to explicitly use these toolchains when building
Firefox.

browser_patches/firefox-beta/BUILD_NUMBER

@@ -1,2 +1,2 @@
-1325
+1326
-Changed: lushnikov@chromium.org Thu May 19 17:56:07 +03 2022
+Changed: lushnikov@chromium.org Tue May 31 10:19:28 +03 2022

browser_patches/firefox-beta/build.sh

@@ -109,7 +109,7 @@ if [[ $1 == "--juggler" ]]; then
 elif [[ $1 == "--bootstrap" ]]; then
   ./mach configure
 else
-  ./mach build
+  MOZ_AUTOMATION=1 MOZ_FETCHES_DIR=$HOME/.mozbuild ./mach build
   if is_mac; then
     node "${SCRIPT_FOLDER}"/install-preferences.js "$PWD"/${OBJ_FOLDER}/dist
   else