docs(test): document DENO_COVERAGE_DIR in help message (#28939)

Author: kt3k

cli/args/flags.rs

@@ -3206,7 +3206,8 @@ or <c>**/__tests__/**</>:
           .conflicts_with("inspect")
           .conflicts_with("inspect-wait")
           .conflicts_with("inspect-brk")
-          .help("Collect coverage profile data into DIR. If DIR is not specified, it uses 'coverage/'")
+          .help(cstr!("Collect coverage profile data into DIR. If DIR is not specified, it uses 'coverage/'.
+  <p(245)>This option can also be set via the DENO_COVERAGE_DIR environment variable."))
           .help_heading(TEST_HEADING),
       )
       .arg(