fix(logs): align error message with var names

.woodpecker/build.yaml

@@ -6,6 +6,7 @@ matrix:
 
 when:
     - event: [push, manual]
+      branch: main
 
 steps:
   - name: Build & publish container

entrypoint.sh

@@ -1,17 +1,17 @@
 #!/usr/bin/env sh
 
 if [ -z "$PLUGIN_GOPRIVATE" ]; then
-  echo "PLUGIN_GOPRIVATE empty"
+  echo "goprivate empty"
   exit 1
 fi
 
 if [ -z "$PLUGIN_LOGIN" ]; then
-  echo "PLUGIN_LOGIN empty"
+  echo "login empty"
   exit 1
 fi
 
 if [ -z "$PLUGIN_PASSWORD" ]; then
-  echo "PLUGIN_PASSWORD empty"
+  echo "password empty"
   exit 1
 fi