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 1ff8b6c3d8

cc @rail

pkgs/applications/networking/browsers/firefox/wrapper.nix

@@ -127,6 +127,7 @@ let
             --suffix PATH ':' "$out${browser.execdir or "/bin"}" \
             --set MOZ_APP_LAUNCHER "${browserName}${nameSuffix}" \
             --set MOZ_SYSTEM_DIR "$out/lib/mozilla" \
+            --set SNAP_NAME "firefox" \
             ${lib.optionalString gdkWayland ''
               --set GDK_BACKEND "wayland" \
             ''}${lib.optionalString (browser ? gtk3)