Use fixed update site for Nebula widgets

[ptziegler]

We used to reference the latest incubation widgets because they weren't included in any stable release. This is no longer the case since the 3.2.0 release.

[merks]

That’s quite an old version. It think that version is broken relative to the latest css changes in the platform.

CC @wimjongman @vogella

[ptziegler]

@merks That's only for the build against the 2024-09 Eclipse release. But I suppose even the job building against the 2026-12 release is using said update site. But WindowBuilder doesn't use any of the CSS widgets, so I don't think that makes a difference here.

[github-actions]

Test Results

     4 files  ±0       4 suites  ±0   2h 12m 27s ⏱️ + 9m 35s
 7 019 tests ±0   6 989 ✅ +1  30 💤 ±0  0 ❌ ±0 
23 680 runs  ±0  23 584 ✅ +1  96 💤 ±0  0 ❌ ±0 

Results for commit bbfb05b. ± Comparison against base commit deca6aa.