Install Firefox from Gentoo binpkg

Unfortunately, even building Firefox with GCC fails:

> 3:30.02 [gecko-profiler 0.1.0] /../lib/gcc/aarch64-unknown-linux-gnu/14/include/g++-v14/cstdlib:79:15: fatal error: 'stdlib.h' file not found
> 3:30.02 [gecko-profiler 0.1.0] thread 'main' panicked at tools/profiler/rust-api/build.rs:104:10:
> 3:30.02 [gecko-profiler 0.1.0] Unable to generate bindings: ClangDiagnostic("/../lib/gcc/aarch64-unknown-linux-gnu/14/include/g++-v14/cstdlib:79:15: fatal error: 'stdlib.h' file not found\n")

Clearly, something is misconfigured, because `stdlib.h` does indeed
exist.  I am not sure what, though, and I am getting tired of messing
with this.

Fortunately, the official Gentoo binary package project has a build of
_www-client/firefox_ for ARM64.  It has a rather different USE flag
configuration than what we did, though, so we have to pull in quite a
few more dependencies.

We can't just add _www-client/firefox_ to `install.packages` because
Aimee OS runs `emerge` with `--getbinpkgonly`, which implies
`--binpkg-changed-deps=y`.  This since we want to build everything
_except_ Firefox locally, the dependency graph is quite a bit different,
so Portage ignores the binary package and will try to build
_www-client/firefox_ from source.

To work around this limitation, we need to install Firefox manually in
the `customize.sh` script in two phases.  First, we install all of its
dependencies in the build root (`/usr/aarch64-…`), but not Firefox
itself, to get binpkgs for them.  Then, we install _www-client/firefox_
in the target root (`/mnt/gentoo`) with the `--getbinpkg` and
`--usepkgonly` flags.

Hopefully, one day I can figure out how to cross-compile Firefox (and it
doesn't take days to build once I do), and we can remove this hackery.

portage/make.conf/wayland.conf

@@ -1 +1 @@
-USE="${USE} -X wayland"
+USE="${USE} wayland"

portage/package.accept_keywords/firefox

@@ -1 +0,0 @@
-dev-libs/nss ~amd64

portage/package.use/firefox

@@ -2,5 +2,14 @@ media-libs/harfbuzz -cairo
 media-libs/libvpx postproc
 media-libs/mesa -llvm wayland
 media-video/ffmpeg openssl -gnutls
-www-client/firefox -clang -telemetry dbus wayland
+
+# Must match USE flags of the www-client/firefox package on the
+# offical Gentoo binhost
+www-client/firefox X clang dbus gmp-autoupdate gnome-shell jumbo-build pulseaudio system-av1 system-harfbuzz system-icu system-jpeg system-libevent system-libvpx system-webp telemetry wayland LLVM_SLOT: 19 -17 -18
+x11-libs/cairo X
 x11-libs/gtk+ wayland
+media-libs/libglvnd X
+
+# Firefox requires a PulseAudio-compatible sound server; we use Pipewire
+media-video/pipewire sound-server
+media-libs/libcanberra udev alsa