CI: do not test Webkit on Linux

The tests work correctly in Epiphany, but not in the WebKit build,
where the native X25519 implementation throws non-standard errors on
importKey (DataError) and generateKey (OperationError).
Patching this would be simply a matter of catching such errors and falling back
to the JS implementation, but since only the CI WebKit build seems to be
affected, we prefer not to relax fallback checks in the context of crypto
operations without issues reported in the wild.

.github/workflows/tests.yml

@@ -106,6 +106,7 @@ jobs:
           npx playwright install --with-deps firefox
 
       - name: Install WebKit # caching not possible, external shared libraries required
+        if: ${{ matrix.runner == 'macos-latest' }} # do not install on ubuntu, since the X25519 WebCrypto implementation has issues
         run: npx playwright install --with-deps webkit
 
       - name: Run browser tests