fix wd-test test coverage reporting

Author: anonrig

.github/workflows/_bazel.yml

@@ -125,6 +125,8 @@ jobs:
         run: |
           bazel --nowindows_enable_symlinks build ${{ inputs.extra_bazel_args }} --config=ci --profile build-win-workaround.bazel-profile.gz --remote_cache=https://bazel:${{ secrets.BAZEL_CACHE_KEY }}@bazel-remote-cache.devprod.cloudflare.dev //src/wpt:wpt-all@tsproject //src/node:node@tsproject //src/pyodide:pyodide_static@tsproject
       - name: Bazel build
+        # Skip build step when running coverage - bazel coverage will build with instrumentation
+        if: ${{ !inputs.run_coverage }}
         # timestamps are no longer being added here, the GitHub logs include timestamps (Use
         # 'Show timestamps' on the web interface)
         run: |

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.