[releng] Fix up usages of tycho-source-plugin

[mbooth101]

Currently you can see this warning about migrating to the new feature-source goal:

[INFO] --- tycho-source-feature-plugin:2.7.5:source-feature (source-feature) @ org.eclipse.cdt.testing ---
[WARNING] Mojo tycho-source-feature-plugin:source-feature is replaced by the tycho-source-plugin:feature-source which offers equivalent and even enhanced functionality

And you can see duplicate invokations of the plugin-source goal:

[INFO] --- tycho-source-plugin:2.7.5:plugin-source (plugin-source) @ org.eclipse.cdt.platform.branding ---
[INFO] Building jar: /home/mbooth/workspace-cdt/org.eclipse.cdt/releng/org.eclipse.cdt.platform.branding/target/org.eclipse.cdt.platform.branding-11.0.0-SNAPSHOT-sources.jar
[INFO] 
[INFO] --- tycho-source-plugin:2.7.5:plugin-source (attach-source) @ org.eclipse.cdt.platform.branding ---
[INFO] Building jar: /home/mbooth/workspace-cdt/org.eclipse.cdt/releng/org.eclipse.cdt.platform.branding/target/org.eclipse.cdt.platform.branding-11.0.0-SNAPSHOT-sources.jar

This change fixes both problems and will ease eventual migration to tycho 3.

[github-actions]

Test Results

     612 files       612 suites   17m 7s ⏱️
10 253 tests 10 230 ✔️ 21 💤 2 ❌
10 216 runs  10 193 ✔️ 21 💤 2 ❌

For more details on these failures, see this check.

Results for commit c93377e.

♻️ This comment has been updated with latest results.

[akurtakov]

GH action fails but jenkins job succeeds so I think it's not issue with the patch, which is otherwise good.