Update io_bazel_rules_scala digest to 15cec88

Author: renovate[bot]

WORKSPACE

@@ -58,9 +58,9 @@ http_archive(
 
 http_archive(
     name = "io_bazel_rules_scala",
-    sha256 = "31dac610fe73d7211ea2083540d085a8f8b582ac8afc3f449173309eda7fc7df",
-    strip_prefix = "rules_scala-b17e9e0580ba28f95559676dd88cd99ec4f08195",
-    url = "https://github.com/bazelbuild/rules_scala/archive/b17e9e0580ba28f95559676dd88cd99ec4f08195.tar.gz",
+    sha256 = "f6de3147a72c990b02652ce8d5be8ad217d31b5a4634136778fcbd1ee2ee1db1",
+    strip_prefix = "rules_scala-15cec888ed65aadba2c909b3080f0083d6eac019",
+    url = "https://github.com/bazelbuild/rules_scala/archive/15cec888ed65aadba2c909b3080f0083d6eac019.tar.gz",
 )
 
 http_archive(