Rakefile: actually build with the base branch merged

!381 (merged) introduced a regression that makes $GIT_COMMIT stick to the pre-merge commit, when it should be updated to the post-merge commit.

As a result, after we've correctly merged the base branch on the host, in the build VM, build-tails resets the build tree to the pre-merge commit:

    git reset --hard "${GIT_COMMIT}"

… which essentially reverts the base branch merge.

Closes #18441 (closed)