[lit] Fix internal env calling env

Without this patch, when using lit's internal shell, if `env` on a lit
RUN line calls `env`, lit accidentally searches for the latter as an
external executable.  What's worse is that works fine when a developer
is testing on a platform where `env` is available and behaves as
expected, but it then breaks on other platforms.

`env` calling `env` can make sense if one such `env` is within a lit
substitution, as in D65156 and D65121.  This patch ensures that lit
executes both as internal commands.

Reviewed By: probinson, mgorny, rnk

Differential Revision: https://reviews.llvm.org/D65697