update to github action actions/upload-artifact@v4 from v3

we'll now get a coverage file artifact for each of the builds. we do two
builds, and the last was likely overwriting the coverage file "artifact" of the
first.

hopefully fixes the test. can't test it locally...

.github/workflows/build-test.yml

@@ -27,8 +27,9 @@ jobs:
       # Need to run tests with a temp dir on same file system for os.Rename to succeed.
       - run: 'mkdir -p tmp && TMPDIR=$PWD/tmp make test'
 
-      - uses: actions/upload-artifact@v3
+      - uses: actions/upload-artifact@v4
         with:
+          name: coverage-${{ matrix.runs-on }}
           path: cover.html
 
       # Format code, we check below if nothing changed.