fix: Update System.env syntax for Gradle 9 compatibility (opensearch-project#1474)

Signed-off-by: Daniel Widdis <[email protected]>

Author: dbwiddis

build.gradle

@@ -294,8 +294,8 @@ publishing {
             name = "Snapshots"
             url = "https://central.sonatype.com/repository/maven-snapshots/"
             credentials {
-                username "$System.env.SONATYPE_USERNAME"
-                password "$System.env.SONATYPE_PASSWORD"
+                username System.getenv("SONATYPE_USERNAME")
+                password System.getenv("SONATYPE_PASSWORD")
             }
         }
     }

spi/build.gradle

@@ -120,8 +120,8 @@ publishing {
             name = "Snapshots"
             url = "https://central.sonatype.com/repository/maven-snapshots/"
             credentials {
-                username "$System.env.SONATYPE_USERNAME"
-                password "$System.env.SONATYPE_PASSWORD"
+                username System.getenv("SONATYPE_USERNAME")
+                password System.getenv("SONATYPE_PASSWORD")
             }
         }
     }