ext/gd: enable avif tests on GH-Actions (php#15850)

When the avif support was first added (phpGH-7026), php-src did not
use GitHub Actions. When we moved to GitHub Actions for CI, the
`--with-avif` option nor the `libavif-dev` dependencies were not
added.

Cirrus CI still runs the tests, and now that we no longer use
Travis CI, this brings parity to test avif on x64 tests on
GitHub Actions as well.

`libavif-dev` package's x86 builds seem to be missing[^1][^2] on older
Ubuntu versions, so this commit only adds it for x64 builds.

[^1]: https://packages.ubuntu.com/jammy/libavif-dev
[^2]: https://packages.ubuntu.com/noble/libavif-dev

Author: Ayesh

.github/actions/apt-x64/action.yml

@@ -39,6 +39,7 @@ runs:
           libsqlite3-dev \
           libsqlite3-mod-spatialite \
           libwebp-dev \
+          libavif-dev \
           libonig-dev \
           libcurl4-openssl-dev \
           libxml2-dev \

.github/actions/configure-x64/action.yml

@@ -28,6 +28,7 @@ runs:
           --enable-gd \
           --with-jpeg \
           --with-webp \
+          --with-avif \
           --with-freetype \
           --with-xpm \
           --enable-exif \