gnu: ungoogled-chromium: Build with ffmpeg@4.2.

Fixes <https://bugs.gnu.org/41987>.
Reported by Jonathan Brielmaier <jonathan.brielmaier@web.de>.

* gnu/packages/video.scm (ffmpeg-4.2): New public variable.
* gnu/packages/chromium.scm (ungoogled-chromium)[inputs]: Change from FFMPEG
to FFMPEG-4.2.

gnu/packages/chromium.scm

@@ -785,7 +785,11 @@ (define-public ungoogled-chromium
        ("dbus-glib" ,dbus-glib)
        ("expat" ,expat)
        ("flac" ,flac)
-       ("ffmpeg" ,ffmpeg)
+
+       ;; FIXME: Change to ffmpeg 4.3 when supported, see
+       ;; <https://bugs.chromium.org/p/chromium/issues/detail?id=1095962>.
+       ("ffmpeg" ,ffmpeg-4.2)
+
        ("fontconfig" ,fontconfig)
        ("freetype" ,freetype)
        ("gdk-pixbuf" ,gdk-pixbuf)

gnu/packages/video.scm

@@ -1126,6 +1126,24 @@ (define-public ffmpeg
 audio/video codec library.")
     (license license:gpl2+)))
 
+;; ungoogled-chromium crashes with ffmpeg 4.3, so stick with this version for
+;; now.  See <https://issues.guix.gnu.org/41987>.
+(define-public ffmpeg-4.2
+  (package
+    (inherit ffmpeg)
+    (version "4.2.3")
+    (source (origin
+              (method url-fetch)
+              (uri (string-append "https://ffmpeg.org/releases/ffmpeg-"
+                                  version ".tar.xz"))
+              (sha256
+               (base32
+                "0cddkb5sma9dzy8i59sfls19rhjlq40zn9mh3x666dqkxl5ckxlx"))))
+    (arguments
+     (substitute-keyword-arguments (package-arguments ffmpeg)
+       ((#:configure-flags flags)
+        `(delete "--enable-librav1e" ,flags))))))
+
 (define-public ffmpeg-3.4
   (package
     (inherit ffmpeg)