Packit: use post-modifications hook to update downstream TMT plan

`prepare-files` action was interfering with spec file update which caused
#2760 .

Also, s/PACKIT_PROJECT_TAG/PACKIT_PROJECT_VERSION/ .

Co-authored-by: Nikola Forró <[email protected]>
Signed-off-by: Lokesh Mandvekar <[email protected]>

Author: lsm5

.packit.yaml

@@ -22,8 +22,8 @@ packages:
 # https://packit.dev/docs/configuration/actions
 actions:
   get-current-version: bash -c 'grep "^const Version" version/version.go | cut -f2 -d\" | tr \- \~'
-  prepare-files: >-
-    bash -c "sed -i 's/^\(\s*\)ref: .*/\1ref: \"${PACKIT_PROJECT_TAG}\"/' ${PACKIT_DOWNSTREAM_REPO}/plans/main.fmf"
+  post-modifications: >-
+    bash -c "sed -i 's/^\(\s*\)ref: .*/\1ref: \"v${PACKIT_PROJECT_VERSION}\"/' ${PACKIT_DOWNSTREAM_REPO}/plans/main.fmf"
 
 srpm_build_deps:
   - make