Build: Split up task in the CI pipeline to ease running outside circleci (#18861)

* build: make sign rpm packages not depend on checking out private key

* build: move commands from circleci config into verify signed packages script

* build: split update and publish of deb and rpm into two scripts

* use files argument for sign and verify packages

* validate files argument for sign and verify packages

* update test publish of deb/rpm readme

scripts/build/verify_signed_packages.sh

@@ -2,6 +2,14 @@
 
 _files=$*
 
+if [ -z "$_files" ]; then
+    echo "_files (arg 1) has to be set"
+    exit 1
+fi
+
+mkdir -p ~/.rpmdb/pubkeys
+curl -s https://packages.grafana.com/gpg.key > ~/.rpmdb/pubkeys/grafana.key
+
 ALL_SIGNED=0
 
 for file in $_files; do