Bug 40841: Set SIGNING_PROJECTNAME=torbrowser in signing scripts

For compatibility with signing scripts on the main branch.

tools/signing/set-config

@@ -2,6 +2,7 @@
 . "$script_dir/set-config.hosts"
 
 bundle_locales="ALL"
+export SIGNING_PROJECTNAME=torbrowser
 
 signed_dir="$script_dir/../../$tbb_version_type/signed"
 signed_version_dir="$signed_dir/$tbb_version"