LT-22298: Fix Patching: set both build numbers

[papeh]

FW_BUILD_NUMBER sets the build number on assemblies; BuildVersionSegment sets it on the installer.

NAnt required base installers to be x.x.x.1; WiX may not. I should test whether having a higher-numbered base build works, as this would allow us to get rid of an otherwise redundant variable.

Add summary documentation to installer workflows.

Change-Id: Icb0fd8934cf21c823a9c1490fa2647960bcb1ac4

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This change is 

[jasonleenaylor]

.github/workflows/patch-installer-cd.yml line 269 at r1 (raw file):

        run: |
          # The github action base-installer-cd will make releases with a tag build-[build number].
          # REVIEW (Hasso) 2025.10: we gave up on targeting Jenkins bases; how much of this is obsolete?

I think the comment is obsolete, we still have to be able to build against a previous release so we can patch our stable and our betas. I think everything in here is needed for our current uses.

[jasonleenaylor]

@jasonleenaylor reviewed 2 of 2 files at r1, 2 of 2 files at r2, 1 of 1 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @papeh)