Work around nextest macro lib compatibility bug

This sets the `RUSTUP_WINDOWS_PATH_ADD_BIN` environment variable in
the multi-platform test job, to work around
nextest-rs/nextest#1493 which is keeping
the test runner from working on Windows.

See also ruffle-rs/ruffle#16342, which gave
the idea for this, and various other projects that have made such
changes, linked inhttps://github.com/nextest-rs/nextest/issues/1493.

.github/workflows/ci.yml

@@ -84,6 +84,9 @@ jobs:
           tool: nextest
       - name: "Test (nextest)"
         run: cargo nextest run --all --no-fail-fast
+        env:
+          # Works around https://github.com/nextest-rs/nextest/issues/1493.
+          RUSTUP_WINDOWS_PATH_ADD_BIN: '1'
 
   test-32bit:
     runs-on: ubuntu-latest