[Build|GH] Add quick-fix to avoid need for EF-infrastrucutre

[HannesWell]

In order to mitigate the current outage of the EF-hosted storage, which will probably continue to block builds for at least a few days (https://www.eclipse.org/lists/eclipse.org-committers/msg01493.html) this implements a quick-fix to avoid the need for said EF infrastructure for the shared GitHub workflows. This is achieved by the following actions:

• Obtain the eclipse-parent pom.xml and the sdk.target project from the GH repository and install it within the build into the local Maven cache
• Revert Tycho to use it's latest release 4.0.12, which is available from Maven-Central

Especially the latter could cause problems for builds that rely on the new features only available in Tycho 4.0.13-SNAPSHOT. Therefore this is currently designed as opt-in feature. But looking at #2870 (comment), only SWT currently needs the snapshot but doesn't use the shared workflow.
After some more testing we could also enable this by default.

[HeikoKlare]

Great workaround, thank you! 🚀

The change looks good and seems to work (as you show at the platform.ui repo). I only found a nitpicky typo, which you might fix in all three workflows when exchanging the organization name for the used action for final submission.

[HannesWell]

Thank you. I'm looking forward too!
The first jobs using this have succeeds in the testing PRs, some are still running but are looking good:

• [Build|GH] Test changes in shared GH workflow eclipse.platform.ui#2940
• [Build|GH] Use quick-fix to avoid need for EF-infrastrucutre eclipse-pde/eclipse.pde#1739

I just made the final adjustments to use the final repo and branch name, so this is ready for submission.
My intention is to submit this, as soon as all jobs have succeeded in the mentioned PRs.