Packit: fix downstream post-modifications action

The downstream TMT plan can use a ref value without double quotes[0], so
the `post-modifications` action in packit config can be simplified as in
this patch. Also, switch the single and double quote usage to be
consistent with the usage in `get-current-version` packit action.

This should help to avoid variable substitution issues noticed in the
previous release [1].

Refs:
[0]: https://src.fedoraproject.org/rpms/skopeo/pull-request/98
[1]: fd38aa8a5e/f/plans/main.fmf

Signed-off-by: Lokesh Mandvekar <lsm5@redhat.com>
(cherry picked from commit 1ea0fb4baf)

.packit.yaml

@@ -121,7 +121,7 @@ jobs:
       - fedora-all
     actions:
         post-modifications: >-
-          bash -c "sed -i 's/^\(\s*\)ref: .*/\1ref: \"v${PACKIT_PROJECT_VERSION}\"/' ${PACKIT_DOWNSTREAM_REPO}/plans/main.fmf"
+          bash -c 'sed -i "s/^\(\s*\)ref: .*/\1ref: v${PACKIT_PROJECT_VERSION}/" ${PACKIT_DOWNSTREAM_REPO}/plans/main.fmf'
 
   # Sync to CentOS Stream
   # FIXME: Switch trigger whenever we're ready to update CentOS Stream via