Support setting R8 dump directory with environment variable

This will allow developers to easily create dumps of R8 compilations (input classfiles, proguard rules, flags, resources...)

Example usage:
$ mkdir /tmp/dumps
$ R8_DUMP_DIRECTORY=/tmp/dumps m -j77 Traceur
This will put the dump for Traceur into the /tmp/dumps directory

If your target has several R8 invocations then dumps for all of these will be generated into the dump directory

Bug: 369931283
Test: R8_DUMP_DIRECTORY=/tmp/dumps m -j77 Traceur
Change-Id: I69be3190c61a98402931767e42184cc01ef25804