ci: build.sh: Add a helper function to create the dist tarball

This helper function does not correspond to a particular GitLab job, it
just logically separates the necessary step of creating a dist tarball
from the RPM build job that takes over.
One notable change here is the need to update git's file index which
causes issues in local container executions which rely on a shallow
copy of the libvirt repo created as:

    $ git clone --local

Even if all changes have been committed, git often complained
otherwise. Updating the index in a GitLab environment is a NOP.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

ci/build.sh

@@ -42,3 +42,13 @@ run_build() {
     test -f $GIT_ROOT/build/build.ninja || run_meson_setup
     run_cmd meson compile -C build $BUILD_ARGS
 }
+
+run_dist() {
+    test -f $GIT_ROOT/build/build.ninja || run_meson_setup
+
+    # dist is unhappy in local container environment complaining about
+    # uncommitted changes in the repo which is often not the case - refreshing
+    # git's index solves the problem
+    git update-index --refresh
+    run_cmd meson dist -C build --no-tests
+}