ci: allow skipping sudo on dockerized jobs

Our "install-dependencies.sh" script is executed by non-dockerized jobs
to install dependencies. These jobs don't run with "root" permissions,
but with a separate user. Consequently, we need to use sudo(8) there to
elevate permissions when installing packages.

We're about to merge "install-docker-dependencies.sh" into that script
though, and our Docker containers do run as "root". Using sudo(8) is
thus unnecessary there, even though it would be harmless. On some images
like Alpine Linux though there is no sudo(8) available by default, which
would consequently break the build.

Adapt the script to make "sudo" a no-op when running as "root" user.
This allows us to easily reuse the script for our dockerized jobs.

ci/install-dependencies.sh

@@ -11,6 +11,17 @@ UBUNTU_COMMON_PKGS="make libssl-dev libcurl4-openssl-dev libexpat-dev
  tcl tk gettext zlib1g-dev perl-modules liberror-perl libauthen-sasl-perl
  libemail-valid-perl libio-socket-ssl-perl libnet-smtp-ssl-perl"
 
+# Make sudo a no-op and execute the command directly when running as root.
+# While using sudo would be fine on most platforms when we are root already,
+# some platforms like e.g. Alpine Linux do not have sudo available by default
+# and would thus break.
+if test "$USER" = "root"
+then
+	sudo () {
+		"$@"
+	}
+fi
+
 case "$distro" in
 ubuntu-*)
 	sudo apt-get -q update