.gitlab-ci.d/buildtest.yml: Install 'file' for the Coverity job

.gitlab-ci.d/buildtest.yml: Install 'file' for the Coverity job

The coverity-scan upload job has started failing as of 30th
September:

[ERROR] Cannot find '/usr/bin/file' command, and no 'file' command is found in the PATH
        Coverity Capture uses this tool to identify the file type of executables.
        Please ensure '/usr/bin/file' is available, or add the 'file' utility to your PATH.

This seems to have broken when we moved our containers from Fedora 40
to 41 -- probably F40 indirectly pulled in 'file' via some other
dependency, but in F41 it does not.

Explicitly install 'file' for the coverity job, in the same way we
already do for curl and wget.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20251017133156.926094-1-peter.maydell@linaro.org>