Fix locale to en_US for datetime functions to be independent from runtime environments

TestUnixTimeFunctions.testDateFormat and TestUnixTimeFunctions.testDateParse tests fail if we run tests on a machine whose system locale is not en_US because some formats of DateTimeFormatter depend on the default locale of the runtime environment.

This change makes format_datetime, date_format, and date_parse functions independent from the runtime environment.

### How to reproduce

```sh
_JAVA_OPTIONS="-Duser.language=ja -Duser.country=JP" mvn test -Dtest=TestUnixTimeFunctions -DfailIfNoTests=false
```