[vcpkg] Fix ci: suggest vcpkg_*_cmake -> vcpkg_cmake_* migration

[autoantwort]

Fixes the bugs reported in #20142

HEAD^^ refers to the parent of the parent (HEAD^2 refers to the second parent)
If the ci is running, the branch is merged into the master branch, so we for example have the following graph:

*   c21dd13cb (HEAD -> master) Merge branch 'document-deprecated-functions'
|\  
| * aa8e5cc56 (document-deprecated-functions) Document deprecated functions from #20142 in the maintainer guide.
|/  
* 574c125d6 [openimageio] Re-fix the usage (#20092)
* fbe07843a [gettext] Remove `SUBPATH`, add iconv linking info (#20090)

The command git diff --name-status --merge-base HEAD^^ HEAD now compared c21dd13cb(HEAD) against the base fbe07843a, so it detects the file changes in aa8e5cc56 and 574c125d6, which is wrong (it always also included the changes of the latest master commit). Now it compares c21dd13cb(HEAD) against 574c125d6(latest master). This explains the suggestion in #20217

[autoantwort]

The wrong suggestion in #19756 was caused by wrongly using grep 🤦

[BillyONeal]

Thanks for poking the bot :)

[wrobelda]

Something is off, still – the bot now fails with a non-zero exit code and no comment left:
https://github.com/microsoft/vcpkg/pull/19828/checks?check_run_id=3633952227#step:6:17

[autoantwort]

Uff yeah grep returns 1 when it finds nothing which is then interpreted as pipeline failure.

[BillyONeal]

Uff yeah grep returns 1 when it finds nothing which is then interpreted as pipeline failure.

Whack a mole continues 👍

[autoantwort]

Hopefully last fix pr in #20236 😅