Fix central creds retrieval

Author: Haarolean

serde-api/build.gradle

@@ -39,8 +39,8 @@ publishing {
             maven {
                 url "https://s01.oss.sonatype.org/service/local/staging/deploy/maven2"
                 credentials {
-                    username sonatypeUsername
-                    password sonatypePassword
+                    username System.getenv("NEXUS_USERNAME")
+                    password System.getenv("NEXUS_PASSWORD")
                 }
             }
         }