fix: increase migration lock timeout to prevent flaky parallel test (#22910)

## Problem

`TestMigrate/Parallel` flakes with:

```
timeout: can't acquire database lock
```

## Root Cause

The test runs two concurrent `migrations.Up(db)` calls on the same
database. golang-migrate wraps every `Lock()` call with a [15-second
timeout](https://github.com/golang-migrate/migrate/blob/v4.19.0/migrate.go#L29)
(`DefaultLockTimeout`). Our `pgTxnDriver.Lock()` uses
`pg_advisory_xact_lock`, which blocks until the lock is available. With
430+ migrations, the first caller can hold the lock well beyond 15s (the
failing test ran for 25.88s), causing the second caller to hit the
timeout.

## Fix

Set `m.LockTimeout = 2 * time.Minute` after creating the
`migrate.Migrate` instance in `setup()`. Since `pg_advisory_xact_lock`
releases automatically when the transaction commits, there's no risk of
a stuck lock — we just need to wait long enough for a concurrent
migration to finish.

coderd/database/migrations/migrate.go

@@ -12,6 +12,7 @@ import (
 	"sort"
 	"strings"
 	"sync"
+	"time"
 
 	"github.com/golang-migrate/migrate/v4"
 	"github.com/golang-migrate/migrate/v4/source"
@@ -101,6 +102,13 @@ func setup(db *sql.DB, migs fs.FS) (source.Driver, *migrate.Migrate, error) {
 		return nil, nil, xerrors.Errorf("new migrate instance: %w", err)
 	}
 
+	// The default LockTimeout of 15s is too short for concurrent migrations,
+	// especially when the number of migrations is large. Since we use
+	// pg_advisory_xact_lock which releases automatically when the transaction
+	// ends, we just need to wait long enough for any concurrent migration to
+	// finish.
+	m.LockTimeout = 2 * time.Minute
+
 	return sourceDriver, m, nil
 }