Make sure we ship regular release builds with proper non-esr mozconfig files

Among other differences there are some tweaks in used mozconfig files and configurations that set regular release from esr release builds apart. We should make sure the mozconfig files we use for building Fenix reflect that.

We got reminded at that fact by looking closer at the fix that landed in https://bugzilla.mozilla.org/show_bug.cgi?id=1308251 and wondering whether MOZ_REQUIRE_SIGNING is set or not. It's not the case for the esr channel but it is set for regular release builds.

We should try to collect the differences and adjust the mozconfig files/build environment for non-esr builds accordingly where needed.