[Firebird] Use ampersand literal in limit offset sub to fix FIRST 1/SKIP 1

When the SQL is substituted, \\& needs to be a literal.

```ruby
"SELECT * FROM EXAMPLES".sub(/\A(\s*SELECT\s)/i, "\&FIRST 1 ")
#=> "&FIRST 1 * FROM EXAMPLES"

"SELECT * FROM EXAMPLES".sub(/\A(\s*SELECT\s)/i, '\&' + "FIRST 1 ")
#=> "SELECT FIRST 1 * FROM EXAMPLES"
```

The breaking change was introduced here: jruby/activerecord-jdbc-adapter@2bb0f49e161ba2bbf01ad937b5178493e4cd0fc0