[compiler-rt] [test] Use the parent process env as base env in tests

Summary:
Right now the lit config builds up an environment that the tests will be run in. However, it does it from scratch instead of adding new variables to the parent process environment. This may (and does) result in strange behavior when running tests with an executor (i. e. with the `COMPILER_RT_EMULATOR` CMake variable set to something), since the executor may need some of the parent process's environment variables.

Here this is fixed.

Reviewers: compnerd, phosek

Reviewed By: compnerd

Subscribers: dberris, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D83486