Export `TMT_SOURCE_DIR` in prepare/finish steps as well

[lukaszachy]

Use case for 'prepare' is to be able to run 'autotools' with patches sources (eg. when it creates helper test executables) and leave 'discover' to just specify the tests.

Current workaround is to have this 'autotools' action as part of the very first test, which makes it problematic for 'run just a single test' (tmt run tests -n name)

There is no clear use case for finish, but it shouldn't hurt to have that variable there as well.