libavif appears to require pcre2, which it can find (indirectly)
through the nativeBuildInputs for native builds.  For cross builds
we need to add it explicitly.

Error which this fixed:

```
-- Found PkgConfig: /nix/store/13a2qvyl3lfcw0i50kfzlry5apjagn9f-aarch64-unknown-linux-gnu-pkg-config-wrapper-0.29.2/bin/aarch64-unknown-linux-gnu-pkg-config (found version
"0.29.2")
-- Checking for one of the modules 'gdk-pixbuf-2.0'
Package libpcre2-8 was not found in the pkg-config search path.
Perhaps you should add the directory containing `libpcre2-8.pc'
to the PKG_CONFIG_PATH environment variable
Package 'libpcre2-8', required by 'glib-2.0', not found
Package libpcre2-8 was not found in the pkg-config search path.
Perhaps you should add the directory containing `libpcre2-8.pc'
to the PKG_CONFIG_PATH environment variable
Package 'libpcre2-8', required by 'glib-2.0', not found
Package libpcre2-8 was not found in the pkg-config search path.
Perhaps you should add the directory containing `libpcre2-8.pc'
to the PKG_CONFIG_PATH environment variable
Package 'libpcre2-8', required by 'glib-2.0', not found
Package libpcre2-8 was not found in the pkg-config search path.
Perhaps you should add the directory containing `libpcre2-8.pc'
to the PKG_CONFIG_PATH environment variable
Package 'libpcre2-8', required by 'glib-2.0', not found
```