Fix issue linking SCIM data to newly-enrolled hosts (#38408)

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**Related issue:** Resolves #37271 

# Checklist for submitter

If some of the following don't apply, delete the relevant line.

- [X] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
See [Changes
files](https://github.com/fleetdm/fleet/blob/main/docs/Contributing/guides/committing-changes.md#changes-files)
for more information.

- [X] Input data is properly validated, `SELECT *` is avoided, SQL
injection is prevented (using placeholders for values in statements)

## Testing

- [ ] Added/updated automated tests
There's [an existing test suite in
mysql/host_tests.go](https://github.com/fleetdm/fleet/blob/c6746e5967a1153ea4ef68888a77f342bab15598/server/datastore/mysql/hosts_test.go#L12102-L12202),
but that file uses the "one main test with a bunch of cases" pattern to
do a single setup call, and the setup doesn't use the read replica. I'm
guessing that simply flipping that switch would probably break a bunch
of tests that don't expect to have to deal with replica lag in their
test data.

- [X] QA'd all new/changed functionality manually
Set up my local dev with replication enabled and a 1 second lag time,
and verified that an authenticated user had SCIM data linked.

<img width="485" height="395" alt="image"
src="https://github.com/user-attachments/assets/c07369b8-a518-4bcc-ab54-08dd268a65ac"
/>

The ultimate test will be a remote server like Dogfood that has
replication.

changes/37271-use-writer-when-linking-scim-data

@@ -0,0 +1 @@
+- Fixed an issue where newly-enrolled hosts would sometimes not be linked to SCIM user data.