murdock-worker: remove old ccache

[crasbe]

The ccache version installed by Ubuntu 26.04 LTS is newer than one shipped in the murdock-worker Dockerfile, so we don't have to ship this old version and install it over the newer version anymore.

Ccache in riotbuild:

buechse@skyleaf:~$ docker container run --rm --tty -i 87a1af264c72 bash
root@e519b0673a6f:~# ccache -V
ccache version 4.12.3
Features: avx2 file-storage http-storage redis+unix-storage redis-storage

Copyright (C) 2002-2007 Andrew Tridgell
Copyright (C) 2009-2026 Joel Rosdahl and other contributors

See <https://ccache.dev/credits.html> for a complete list of contributors.

This program is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free Software
Foundation; either version 3 of the License, or (at your option) any later
version.

Ccache in murdock-worker:

buechse@skyleaf:~$ docker container run --rm --tty -i 3c9d322ead15 bash
root@437e60a4532b:~# ccache -V
ccache version 4.7.4
Features: file-storage http-storage redis+unix-storage redis-storage

Copyright (C) 2002-2007 Andrew Tridgell
Copyright (C) 2009-2022 Joel Rosdahl and other contributors

See <https://ccache.dev/credits.html> for a complete list of contributors.

This program is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free Software
Foundation; either version 3 of the License, or (at your option) any later
version.

[miri64]

Please rebase.

[crasbe]

Thanks for the review :)