firefox-wrapper: stop creating new profiles when the binary path changes

With a recent change to firefox (that landed in 67) a new profile is
created whenever the install location changes. Since our install
location (the binary path) always changes when we do a new build it is
rather annoying.

Setting the environment variable `SNAP_NAME` to `firefox` is supposed to
workaround the issue.

related to #58923

Woarkound taken from https://github.com/rail/nixos-configs/commit/1ff8b6c3d8b98d0695045eedb97ce135025c36a1

cc @rail