fix wd-test test coverage reporting

Author: anonrig

build/wd_test.bzl

@@ -282,11 +282,14 @@ _wd_test = rule(
         ),
         # Source file
         "src": attr.label(allow_single_file = True),
-        # The workerd executable is used to run all tests
+        # The workerd executable is used to run all tests.
+        # IMPORTANT: cfg must be "target" (not "exec") for coverage instrumentation to work.
+        # When cfg="exec", the binary is built for the execution platform without coverage
+        # instrumentation. Coverage instrumentation is only applied to target platform builds.
         "workerd": attr.label(
             allow_single_file = True,
             executable = True,
-            cfg = "exec",
+            cfg = "target",
             default = "//src/workerd/server:workerd_cross",
         ),
         # A list of files that this test requires to be present in order to run.