fix test coverage workflow

Author: wjakethompson

.github/workflows/test-coverage.yaml

@@ -50,8 +50,9 @@ jobs:
                                    "src/stanExports_test_lcdm.h",
                                    "src/stanExports_test_dina.h",
                                    "src/stanExports_test_dino.h"),
-            install_path = file.path(Sys.getenv("RUNNER_TEMP"), "package")
+            install_path = file.path(normalizePath(Sys.getenv("RUNNER_TEMP"), winslash = "/"), "package")
           )
+          covr::to_cobertura(cov)
         shell: Rscript {0}
 
       - uses: codecov/codecov-action@v4