Do not reuse a server constant built in another scope.

[ianton-ru]

hostName captured Context::isDistributed from the first UNION branch and was then folded on a cluster branch, so the GROUP BY headers no longer matched.

Solved #1181

Changelog category (leave one):

• Bug Fix (user-visible misbehavior in an official stable release)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Fix UNION of a local query and cluster() (or another cluster function) throwing NUMBER_OF_COLUMNS_DOESNT_MATCH or THERE_IS_NO_COLUMN on GROUP BY hostName. A server constant built on the local branch was reused for the cluster branch and folded to the initiator value. Each scope now builds its own value, so shards report their own hostName, serverUUID, tcpPort, and the other server constants.

Documentation entry for user-facing changes

...

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[github-actions]

Workflow [PR], commit [60d1ce0]