Build the test binary with the test run's tags

Why

The integration tests run the real vole binary, but each test harness built it with a plain go build. Tags passed to go test, such as sqlite_fts5, are not inherited by that child command, so a tagged test run was exercising an untagged server. On a checkout without libsqlite3 headers this showed up as two failures:

• TestRebuildIndexAfterDelete: SQLite has no FTS5 module
• TestPushRefreshesTheIndexBeforeTheNextRead: the push did not refresh the index

Both pass once the child binary is built with the same tags as the tests.

What changed

Read the -tags setting from the test binary’s build info and pass it to the child go build in both harnesses:

• server/integration_test.go
• server/ssh/sshserver_test.go

When the test run has no tags the helper returns nothing, so the default build is unchanged.

Verification

• go test -tags sqlite_fts5 ./server/... passes
• go test -tags sqlite_fts5 ./server -run 'TestRebuildIndexAfterDelete|TestPushRefreshesTheIndexBeforeTheNextRead' passes
• ./vole workflow check