Allow $LEIN_JVM_OPTS to override -Dfile.encoding

This allows the JVM to be run with an encoding other than UTF-8; my own use-case for this is running tests to check if I'm forgetting to specify UTF-8 in any character encoding/decoding calls.

Here's a regression test of the behavior:

```bash
$ LEIN_JVM_OPTS='-Dfile.encoding=ASCII' bin/lein run -m clojure.main -e '(System/getProperty "file.encoding")'
"ASCII"
```

`(java.nio.charset.Charset/defaultCharset)` should yield `#<US_ASCII US-ASCII>`.